The same thing happens occasionally on my system...usually when I cold-start from a power-off state. My monitor is an LG widescreen and I always leave it in standby, so it comes up when the PC starts.
I'm pretty sure it's just the monitor auto-adjusting the image size, which is a bit iffy since the POST is just some white text on a largely black background, i.e. no clear way for the monitor to really see where the borders end and the actual "image" begins. So on a cold start, my BIOS text fills the entire screen but on subsequent restarts (when the monitor stays on) it gets smaller with black edges, like what you describe.
In short, I think it's a normal occurence due to the way monitors work, and has nothing to do with your PC itself.