The New ranks do indeed push the Sidebar to a wider extent, but not much wider than when someone uses their Xbox Live ID.
Larger limits for Avatars may be worth concidering, but you have to remember that if everyone has large avatars the forum takes that much longer to load.
Well spotted though ^^
EDIT: P.S. your' sig is breaking the forum, please reduce it in width ^^
(yours is 700px wide and the limit is 575px)