Introducing: My two cents!
After assembling my own three piece boot, at the same scale I made my helmet and torso, it seemed really big. But after looking at reference images, it seemed appropriate. I just needed to figure out a way to get it to fit right, since if I stood inside the boot, (with the sole of my boot on the ground/bottom of boot) it came up really high on the back of my ankle, and moving in it would be really awkward.
So I gave it some thought, and figured that I'll get a much better fit if instead of having the internal shoe on the bottom of the boot, I would keep it as close to the top of the boot as possible. I still have yet to find a way to get that additional height (inside the boot and below my shoe, shown in the blue marked area here) but as you can see, the area which needs to flex (pointed out in green) is much closer to my ankle, vastly improving mobility. It'll also have the added pro of giving me at least an extra inch of height! Once I figure it out, anyway...