I'm not seeing this when I go to your site. I tried in both FF 3.6.3 and IE8. I don't think it has to do with your css or anything.
I would guess it is something to do with your browser. If you're using FF, sometimes you can accidentally zoom in on a page by holding ctrl and hitting your mouse wheel. I believe it will remember and reset the zoom level you use for every page so something like that could be happening.