It's been said that the spaghetti westerns made Eastwood a star, but it was Dirty Harry that made him a "superstar". I'd say that had something to do with why Magnum Force fared a little better at the box office than Dirty Harry, just because of all the "buzz" about who this Eastwood guy was, and hearing about Dirty Harry all the time must have gotten all those folks who still hadn't seen him into the theater to finally see what all the talk was about. Plus, you had fans from the original film that were excited to see a sequel.