Why is Bucky's movie contract so much bigger than Captain America's?

Actor Chris Evans recently announced that his contract with Marvel is for six movies, and that he might take a break from playing Captain America after it's done. We didn't think anything of it... until today, when Sebastian Stan, who played Bucky in the first film, suddenly announced his contract is for nine films. » 3/13/14 5:30pm 3/13/14 5:30pm