If you're talking about the + icon in the title bar, then ya, its behavior is odd. And inconsistent enough to be flagged as buggy, especially with Mac OS 10.5 from what I've noticed. I believe it is supposed to be content-sensitive, but its behavior with some windows is completely puzzling. For instance, I can get it to toggle between 3 or 4 "size" states on some windows instead of the two you'd expect.
I believe you can get third-party programs that will allow you to do a real maximize (expansion of the window to occupy the full screen, excluding the menu bar).