Those videos are great, I use them all the time to get ideas on how, where, when, how, to use monsters. They're great to get an overall understanding of how different monsters and races have been treated during the 40+ years of history of D&D across the editions. What I would really like is them mentioning the source of what they're saying each time, I think that would bring their videos to yet another level.

That being said, in my points I mentioned that the mimic's polymorph seems to be a type of magical effect and, if that's the case, why are we even talking about size compression and all of that? Why are we trying to understand magic through the lens of the physics of our world? When a druid uses wildshape no one is saying "actually, they would come too dense to move around because their mass is conserved" or "well, they can only turn into something roughly their same volume"; the same goes for any other type of transmutation ability or spell. No! We just agree it's something magical and that it goes beyond the laws of physics of our world.

And the last point is the one that supersedes all of them: the DM can use the statblock of any creature as a template to create something new that fits their needs. If they need a mimic about the size of a warhammer to keep that subplot going, let it be. It's far from being something game breaking, and there's even an official monster (the juvenile mimic) that is basically what they need.