Also, what do you mean "too wide to fit" ? Quicktime automatically adjusts its full screen zoom to fit your screen so there should never be an issue here. Even if you tried to watch a 1080p movie on a Macbook, it would shrink the movie's resolution down to match that of the Macbook (with black bars of course, unless you choose "Panoramic" or "Fit to Screen" zoom mode).

I think you are looking for a hint where a problem does not exist. It seems like user error to me.

I'm with you on the confusion, but I have (rarely) got a few videos that were ripped strangely and ended up with black borders on both the sides *and* the top/bottom -- in order to get those to frame nicely fullscreen, some window-resizing could help. Or the scrollwheel method I mentioned below.