That's probably because it’s a poorly designed site/page, or in this case, WordPress theme. I just checked Doug's blog and increasing font size (using Ctrl-+, or Cmd-+ for a Mac) works quite well until the size of the browser gets too small. If your browser is already maximised i.e., it cannot be widened to accommodate the new size, then indeed that's a problem. Doug should perhaps choose a theme that uses flexible/flowing width for its elements.
--ravi