Whenever Big Baby played Center, unless the opposing center was also undersized, he used to struggle.
Defensively he didn't do too badly (he used his body well and took charges nicely) but on offense he often really struggled.
Because of his lack of size and lack of elevation he did get his shots blocked a LOT (commentators even joked at times that he ranked among league leaders in shot attempts blocked) and there were times when I did see him get blocked 3 times on a single posession.
Sully has the same physical limitations but he seems to do a better job at using his body and environment to avoid shotblockers.
Another problem with Baby is that his shot selection wasn't great. 2/3 of his shots were long midrange jumpers which he hit at a passable (but not spectacular) rate of 38%. He had a tendancy to force those shots at time.
Sully also (so far) has been a far better rebounder. Baby only had a rebgound rate of about 11.5%, which is around on bar with what Bass averaged last season for us. Bass isn't a poor rebounding, but he's not a great one either. So far in the preseason Sully has been an excellent rebounding, especially on the offensive end (aroudn 3 per game in ~ 20 minutes).
Both guys have their pros and cons, with Baby being the better defensive player and Sully so far being the better offensive palyer and the better rebounder. Worth noting though that Big Baby had been in the league about 2 or 3 seasons at that stage, while Sullinger is a rookie and has only played 5 games with the team thus far. It'll be interesting to see if Sully's rebounding improves as the season goes on.