No, the file size could be bigger because there are things in the files to prevent bugs and glitchs. Those bugs are not in the xbox so they don't need the extra memory.
Usually file size is based on what the compiler can do, so my guess is the Xbox just has a significantly better compiler than the other two platforms. Or maybe its the size of the machine word. The BSA's are by far the largest files. The format may be more compact on 360, or else maybe someone forgot to use the -optimize flag on the other platforums