From what I've seen, and read, I think the eastern ground hogs are slightly larger on avg., but not by much. Probably more from diet than anything else. Here's a few western rock chucks for comparison:
All in all, I think they are pretty close. The heaviest rock chuck I ever actually weighed was 12 pounds, but I've seen a few that I didn't weigh, that were definitely bigger.
Im guessing that the eastern woodchucks are a bit larger than western rock chucks basicly because of their diet. Ive seen some pretty big rock chucks in my day though.