WTF happened to Michael Shanks between Season 2 and 3?

And for that matter further into the show -- is it that he just started working out or what? I'm rewatching the show right now, just got to The Devil You Know -- and it's not something i really noticed right off the bat, but Daniel/Michael Shanks looks drastically different now. and i know it's more so in the later seasons. Idk how to describe it but his face just looks completely different, like i recognize its him, but at the same time... idk.