Bo doesn't have to bulk up, but he will gain more natural man strength in the next few years, which will hopefully help him play his style without getting injured.
This happens all the time with rookies, because they're not physically mature yet.
The physical rigors of playing against men multiple times a week takes a toll. They have more nagging injuries than they're used to, and the hits they take do more damage. They're not used to that being the case, because they're used to playing against teenagers.