In series you see that the dwarf Vamana becomes a giant and puts his third step on Bali's head. In the series he is a giant and immediately kills Bali (giving him Moksha) yet in the sb I noticed that he was in normal dwarf form when he put his foot on Bali. Which version is correct or are there multiple versions?
By the word ‘sb’ in your question, I am assuming that you mean Srimad Bhagavatam. But, nowhere in Srimad Bhagavatam is it mentioned that Vamana was a dwarf when he placed his foot on Bali Maharaja. Also, as pointed out by other users in the comments section, Bali Maharaja was neither killed nor given Moksha. He was simply sent to Sutala Loka, where he enjoys kingship till today.
Srimad Bhagavatam Canto 8 Chapter 18
29: S'rî Bali said: 'We welcome You. My obeisances unto You, oh brahmin. What can we do for You? Oh noble soul, in my opinion You are the austerity of the brahmin seers in person.
30: The arrival of Your lordship today at our residence, satisfies all our forefathers, it purifies the entire family and completes the sacrifice we are performing now!
31: Today oh brahmin son, my fires of sacrifice are properly served according to the injunctions. Oh, by the water that washed from Your lotus feet all my sins are destroyed and by Your small feet the earth is purified.
32: Whatever it is that You desire, oh brahmacârî, You may take from me, be it a cow, gold, a furnished residence, palatable food and drink or else a brahmin's daughter, prospering villages, horses, elephants or chariots, oh best of the worshipable ones. As far as I am concerned You may have whatever You wish.
Srimad Bhagavatam Canto 8 Chapter 19
16: [The Supreme Lord said:] Someone like your Majesty I ask for a little bit of land. Oh King of the Daityas, from him who can be so generous in his charity I ask three footsteps to the measure of My reach.
17: There is nothing else I desire from you, oh munificent King, oh master of the universe. May the one of learning not suffer any want and receive by donations as much as he needs.
[19.18 to 20.13 are verse in which Bali Maharaja says that he can even give an entire continent to this Brahmin, but Lord Vamana gives a short discourse to Bali Maharaja about how one must be satisfied with what he has. Thus, Bali Maharaja decides to give the Brahmin what he wants. But his Guru Shukracharya advises him against doing so, knowing well that it is Vishnu in person.]
Srimad Bhagavatam Canto 8 Chapter 20
14: S'rî S'uka said: '[Bali,] the highly elevated and divinely inspired character thus being fixed on truthfulness then was cursed by his guru for being such a disrespectful and obstinate disciple:
15: 'You who so stubbornly consider yourself learned going against my instructions, have in your impudence towards me proven yourself as a shameless ignoramus. Someone like you will soon lose all his opulence!'
16: [Even] being cursed this way by his guru he, as a great personality did not abandon his truthfulness and donated after first offering water and duly having worshiped Vâmanadeva [the land he had promised].
17: Vindhyâvali, Bali's wife, that moment came forward being decorated with a pearl necklace and brought a golden pot filled with water to wash the Lord's feet.
18: He, the worshiper of the most beautiful pair of feet, personally washed them and took in great jubilation the water on his head that purifies the entire universe.
19: That moment a shower of flowers was released by all the inhabitants of the higher worlds: the demigods, the singers of heaven, the scholars, the ones specially gifted and the venerable ones. They all most pleased hailed the rectitude of what the Asura king had done!
20: The residents of heaven, the apelike ones and the ones of superpower sounded thousands of drums and horns again and again and declared: 'What by Him, this great personality, has been achieved was a most difficult thing, for he delivered the three worlds to his opponent [Vishnu]!
21: Then the dwarf form of the Unlimited Lord began to expand most wondrously to the entire expanse of the threefold of matter: He stretched Himself out in every direction over all the land, the sky, the planetary systems, outer space and the seas and oceans where the birds and the beasts, the humans, the gods and the saints were living.
[22-33 talks about the gigantic form of Vamana in which the whole universe was embodied.]
34: Making the second step He covered all the heavenly places and for the third step not a single spot of land remained, for Lord Urukrama with His stepping now had reached farther than the farthest place beyond Maharloka, Janaloka and Tapoloka.
Srimad Bhagavatam Canto 8 Chapter 21
1: S'rî S'uka said: 'He who appeared on the lotus [Brahmâ] observed from the place of truth [from Satyaloka] how the light of his abode had been covered and had faded because of the effulgence of the Lord's toenails and thus, oh god of man, he approached Him together with vowed brahmacârîs like the sages headed by Marîci and with Sanandana and the other Kumâras.
5: Lord Brahmâ and the others, they who were the predominating deities of the different worlds, were of the greatest respect for their master and collected with all their followers the necessities for worshiping the All-powerful Soul who had returned to His original size.
Srimad Bhagavatam Canto 8 Chapter 21
28: Bereft of his luster he, the one so magnanimous and celebrated, oh King, remained determined as ever. Unto him thus being bound with Varuna's ropes the Supreme Lord Vâmana said:
29: 'You have given Me three steps of land, oh Asura. With two I occupied the complete surface of the earth and now you owe me a third one. As far as the sun, the moon and the stars can shed their light and as far as the clouds are pouring rain, you own all the land. In one step with My body occupying the sky in all directions I have covered the entire sphere of the earth [Bhûrloka] and with the second step I before your eyes occupied the higher worlds you own.
Thus, from the sections highlighted in bold, we find that the Lord’s gigantic divine body extended up until Satyaloka.
Then the Supreme Lord orders Bali Maharaja to go and rule in the realm called Sutala Loka, promising him that he himself would guard that realm with his Chakra. The Lord also blessed with the boon, that in the Manvantara of Savarni.
Srimad Bhagavatam Canto 8 Chapter 22
1: S'rî S'uka said: 'Thus having run into trouble with the Supreme Lord oh King, Bali the Asura king, was despite his awkward position an unperturbed soul who replied positively with the following words.
2: S'rî Bali said: ‘If, oh Lord Praised in the Verses, Your Goodness thinks that what I have promised proved to be false, oh Greatest of the Gods, then let me, in order to be just in this matter and not to have turned to cheating, offer You my head to put the third step of Your lotus feet.’
[3-30 are verses wherein Prahlada Maharaja thanks the Lord for having destroyed the pride of his grandson, and, Vindhyavali, the wife of Bali Maharaja, and Brahma beseech the Lord to not chastise a great soul such as Bali Maharaja.]
31: [The Supreme Lord said:] Through Me he has achieved a position that even for the immortal ones is most difficult to obtain; during the time of Sâvarni Manu he will become the Indra enjoying My full protection.
32: Till then he may go and live in Sutala [the place created by [the heavenly architect] Vis'vakarmâ, where it because of My supervision has been made impossible for the inhabitants to suffer psychically or physically any weariness, exhaustion or defeat.
33: O Indrasena (Bali Mahârâja) please go now, O ruler. May there in Sutala, the place so desirable for even the denizens of heaven, for you surrounded by your people, be all auspiciousness.
34: None of the controllers of the worlds will be able to overrule your command there, not to speak of the common man, for I with my cakra will personally take care of all the Daityas who defy your rule.
This is also mentioned in SB 5.14.18
Below the planet Vitala is another planet, known as Sutala, where the great son of Mahārāja Virocana, Bali Mahārāja, who is celebrated as the most pious king, resides even now. For the welfare of Indra, the King of heaven, Lord Viṣṇu appeared in the form of a dwarf brahmacārī as the son of Aditi and tricked Bali Mahārāja by begging for only three paces of land but taking all the three worlds. Being very pleased with Bali Mahārāja for giving all his possessions, the Lord returned his kingdom and made him richer than the opulent King Indra. Even now, Bali Mahārāja engages in devotional service by worshiping the Supreme Personality of Godhead in the planet of Sutala.
Thus, from the Srimad Bhagavatam, we can conclude that the Lord was indeed in his gigantic form, when placing his third foot on the head of Bali Maharaja.
Yes he became giant when he sent king in patala see the link https://vanisource.org/wiki/SB_8.20.21?t=hl#terms=Bali%20Mah%C4%81r%C4%81ja