Is it me or did Aragog the Acromantula (Hp 2) look so much bigger then in Harry Potter and the Half-blooded Prince when he died? amirite?

Yep, that's what I'd say. People shrink when they get older. Why not acromantula?

well, the movies are inconsistent because they went through different directors. the first two were made by the same guy, the third a different one, and i think the rest by another? so we can't expect the movies to follow along the same guidelines, although the new directors should try and keep certain aspects similar.