Wow, thanks for the comments everyone - I didn't expect so many people to reply - I really appreciate it!
sitekeeper, thanks for the screen shot. I'm on IE 6, text size Medium and the fonts look the way they were intended to be. Generally, text size set by the browser is ignored when using CSS - I'm unsure why all fonts set to 8pt are loading as 10pt. I'll have to play around with it.