Well..... He's only fought a handful of them. He barely beat Arlovski as well and might not have had it not been for the timekeeping error in the first round. Arlovski and Gustafsson are the only opponents taller than him that I can think of. And let's be honest Rumble is a natural LHW. He was killing himself to make Welterweight quite literally.
I guess being short and stockier means he packs more power than the taller lankier opponent. Also being shorter makes it easier for uppercuts and in fighting i guess
