Someone might re-size your site because the design forces them to.

I organize my windows so that my browser takes up the left half of my screen - so it's 960 pixels wide. Pretty big. I never make it any wider than this unless I'm looking at pictures, or forced to by a bad design.

This site exemplifies a bad design that would force me to re-size my browser. I had to make my browser wider to get rid of a horizontal scroll bar on a code block. Nobody reading code on a website ever wants to scroll the code window side to side!

Even though there was plenty of horizontal space for the code blocks to be displayed without scroll bars, this page's design favored blank space on either side of the code blocks over the code itself.

When I have the browser wide enough to display all of the code, it may as well be taking up my entire screen. The remaining horizontal space isn't usable for anything.

I see this problem with code samples on webpages all the time.

Web designers today seem to be operating under the delusion that their site will be the only thing on a user's screen.