why are quarterbacks dressed in jerseys that are not as tight-fitting as other players

[AstroManLuca]

Quarterbacks often wear an extended set of pads that protects their midsection in addition to their shoulders. I think they might be more vulnerable to hits to the body than other players. Also, the starting quarterback is very important to the team and it's important that he doesn't get injured, so he's going to wear extra padding.

EDIT: Look here http://entertainment.howstuffworks.com/fb-equip8.htm