If you want realism, organisms that large could never exist in Earth gravity at all. They'd be too heavy to move and would suffocate under their own weight. So movies like this demand suspension of disbelief.
That's only fractionally correct. It assumes the organisms have the same body structures as other terrestrial organisms. The same type of bones, ligaments, cartilage, tendons, etc. But they're not terrestrial organisms. Not even remotely. Hell, they probably didn't even originate from our universe
Trying to claim realism in this regard is a complete joke. Yes, you have to suspend your disbelief here because there's literally zero
science behind their existence. You can argue about whether a portal to another universe/dimension exists, or even if other universe/dimensions exist, but since in the movie they do exist without question, you can't dismiss that those other universes/dimensions obviously have things in them that defy our own limited, minute understanding of just
the things on our own planet.
That's completely different from arguing about the realism of building a giant wall or huge, pointless robots. Those are all based on the "reality" of our world, using our physics, and our understanding of how things work. The kaiju are completely exempt.