As in, fills it most of the way, or doesn't really fill it at all?
If it's most of the way, it could just be an aspect ratio difference. Not all 1080p content (movies, especially) are exactly 1920x1080. There are a lot of movies that shoot in different aspect ratios just to p!$$ you off, err, I mean, for artistic reasons, and as such, don't fit into the 16:9 aspect ratio (aka, 1.78:1) of traditional 1080p. They might be any number of formats. Take a look at
http://www.digitalcinemasociety.org/downloads/AspectRatiocharts.pdf for some examples.
If it's not even coming close, it may be that your files are ripped/encoded from a 1080p source, but were encoded at something much lower for the sake of file size. For that one, you'll have to take your complaint to the source of your files or find the setting you need to stretch the picture to your full screen size. I've never used XBMC, but the native Xbox360 interface gives you this option in the playback controls.