Doesn't matter if he bulks up. He is big enough already. He needs to be willing to use that size and so far he hasn't figured that out. Bulking up would just make him an even larger softie.
Yeah, he's not willing at all to go in and take the body. Maybe some of it is that he's too position minded and doesn't want to blow his coverage coming back, we saw a similar problem with Filppula who wouldn't engage at all offensively while at center, but was capable of getting in on the play more and putting up better numbers on the wing. Maybe moving Sheahan to wing would encourage him to take that extra step and take the body, knowing he doesn't have to worry so much about his defensive coverage. I doubt it, but maybe.