Can't get it fullscreen in lower resolutions

[Bad doggie. DOWN.]

I'm trying to run this game on my relatively crappy laptop (I don't have my desktop with me at the moment) and I need to turn down the game resolution to get a decent framerate. The problem is that the game won't go fullscreen at lower resolutions. Instead, it just covers part of the screen, which makes everything look so small as to make the game unplayable. I need it to stretch at lower resolutions to fullscreen so I can actually see what's going on in the game without being distracted by my desktop in the background and without having to squint the whole time. Help?