@shinpanman:
Yeah, that's kind of strange, Hancock always seemed rather large, but Mihawk has been normal size in all of his appearances before now.
Anyway, judging from Ace's reaction throughout the explanation, I'd say he already knew about it. I'm still trying to wrap my head around all of this though. What I'm really curious about is why the Marines even cared about Roger's son. A baby can't be guilty and they couldn't have known he'd grow up to be a dangerous pirate. Seems kinda ridonculous to me.
More or less the same reason they cared about a certain eight year old. She was connected to someone who was, to them, extremely dangerous. Plus by all indications (primarily Franky's flashback and Tom) it seems the government made it a point to go after anyone even remotely connected to Roger, no matter how indirectly. This could be a result of Roger ensuring that his crew wasn't executed with him. The Marines wanted/needed to feel they got everyone they could.
I agree with Arei. Being the child of a criminal certainly seems to carry a particular stigma in the One Piece world. Nico Olvia was worried about Robin suffering because of this. Ace assured Garp that he and Luffy could never be Marines because of their bloodlines. Even the Franky Family thought Franky a fine choice for pirate in part because of his father being one.