wwe lost the "heavyweight" part of the belt a few years back. now it does not matter how big or how small you are, what matters is how you are able to get over with the fans and how good your wrestling skills are.
look a mysterio and punk, they are not even close to fitting the heavyweight persona, but they are great wrestlers and the fans love them, so thats why they were the champions
but there are exceptions, like jeff hardy, he can can put on a great wrestling show, and the fans go crazy over him, but he has a very terrible track record with failing wellness tests and just plain having a bad attitude, so i dont think it would be a good idea for him to be champion