I don't use undersized to label sully anymore. BC is not. But he does play under the rim which is not ez for pf/c to play effectively. Robinson plays above the rim though he is similar in height and size. Guys like faried above the rim. Barkley, only 6'6 but had like 7'2 wingspan and can jump out of the gym.
At worse sully will be a consistent 12/8 guy and best 15/10. Even at 12/8 he will be a very good starter BC he does alot of other non stat things that help a team win