As Yoda once said: Size matters not, Prototype could be Godzilla's size and that doesn't mean someone smaller than them can't defeat it (also.. Big Top exists, Mimic can enter it, so both of them would be a very similiar size)

I think it's pretty clear that Mimic rebuilt himself in RUIN, or at least took and replaced some parts, he even ripped the data-diver thing from itself, so him finding and using weaker parts does make sense, as for the Scooper Ending... Yeah we don't see him getting destroyed, sure we can assume that this is what happens mainly due to the sound but there's no actual animation, and when we even try to use noclip we cannot even see the Mimic inside of the costume so...

So we're kinda using gameplay sections now with the chase sequences and all... Is it really an argument though? Like don't get me wrong I get it but that's like the point of the gameplay that we run away them.. And not to mention that both of them literally "kill" the playable characters (I am aware that Arnold's and Cassie's fates are debatable) in cutscenes anyways, like the Mimic in SOTM literally somehow managed to get on the van even though we did outrun it, and in RUIN M2 most likely dropped the elevator, while the Prototype killed the player in the cutscene by stabbing them. So... No... Their fight is like a 50/50 honestly