I just took a quick look and in the shrunk version you overwrite bS, fS, pS all the time and in the other version you have separate objects for boxDef, groundDef etc. Perhaps that has something to do with it?
I read on the Box2D forums, that the *Def can be reused. But maybe i did something wrong somewhere... :s
On a sidenote: Is the shrunk version only shorter variable names? In that case I think you should ditch the shrunken version. Why would you want to change your code to something that´s much harder to read?
It makes the final Class file smaller
I'm secretly practicing for Java4K!
EDIT: PolygonShape has a .clone() so thats not the problem, as I clone it before I reuse it.
EDIT2: On the other hand, maybe reusing variables only work in the C/C++ version