It's got nothing to do with story, just general shape.
A T-Rex, especially the kinda pudgy one that Trypticon transforms into, has VERY wide-set hips. The legs are around the height of his body, but they're set at the side of his torso, not the bottom. If he's smaller, it's just due to what he literally is, nothing about the toy itself. If I made a guess, I'd estimate this thing is gonna be $150-$200.