You can use the software zoom by adding it as a switch from the command line
mplayer some_video.file -zoom -fs
You will notice a performace loss since it's the CPU scaling the image, rather than the graphics card, but on my 500mHz 64MB RAM laptop, it's still very watchable. Might be worthwhile adding the -cache <some_value> switch if it's laggy, as well as the -framedrop (or even -hardframedrop) ones to improve playback. Check out the man page for specifics of these switches.