I think that "body weight" is a single compound noun being modified by "smaller". (Note that "body" is never an adjective in any case). More context would make it easier to be sure, though.
Well, body weight is the 'weight of the body'. It's a compound noun, as Istarion, pointed out. It's identity as a single unit is easily picked up on if you listen to a native speaker produce the phrase.
It is modified by smaller, which also modifies size. Unlike Parla, I find nothing 'un-native' about the sentence as written. We could certainly dress it up a bit, but it still sounds to me like a perfectly natural way for expressing the intended idea.