It is dependent on a few things. First, most web pages were designed for a
resolution of 800x600. If you are using a higher resolution than that, some
sites will not fill the browser screen.
Something you can try other than changing resolution. Open Internet
Explorer, I assume that's the browser you are using, go to Tools, select
Internet Options, on the General tab, click "Accessibility," place a check
next to "Ignore font styles specified on Web pages" and next to "Ignore font
sizes on specified on Web pages," click ok and see if that helps. Note,
this may effect some sites and even some system items in an undesirable way.