If Merged Zamasu didn't get that much stronger (which they could sense easily) then they wouldn't have even suggest fusing. Logic 101Lukmendes wrote:At no point in the conversation the characters say something among the lines of "That power up Zamasu did made him too strong, we need fusion to beat him", or any line even mentioning he got stronger, and the way the conversation goes, we have Gowasu saying that Zamasu was becoming unstable and that hitting him harder would help to defeat him, and then Goku suggests the fusion otherwise they can't beat him, which makes sense considering he broke his arms with that kamehameha and he didn't seriously hurt Zamasu, so basicaly it seems they would have tried fusion to deal with him anyways, since, again, no point in the dialogue is suggested he got stronger, so again, how much stronger he got is pretty moot.zarmack wrote:Merged Zamasu himself flat out states that he needs to get stronger after the fight with Goku, that's why he use the lightning on himself and gets the purple arm. If he didn't get any stronger from that then Goku and Vegeta wouldn't have felt the need to fuse.
There is literally no reason to assume Merged Zamasu didn't get much stronger after gaining the purple arm. If anything, Merged Zamasu at full power in the anime should have been somewhat stronger than Vegito Blue, since Goku Black in his prime was stronger than both SSB Goku/Vegeta added together and Future Zamasu was at least RoF SSB level in power.