"default" size really is, as noted, just so much fantasyland malarkey... while gecko and webkit implementations ignore the OS system metric, you can still change their 'base' font size, and of course Opera and IE actually OBEY the system metric, so the default size can be almost anything!
"system metric" refers to the DPI/default font setting of the host OS. I know for Apple users this is an alien concept with their "what do you mean you want to change the size of the menu text" our way or the highway attitude, but on windows since around... 1989 there were two default settings, normal and 8514 -- this led to the creation of the "large font/small font" notion, later calling them '96dpi' and '120dpi' (which is inaccurate since nobody actually adjusts to the display) and under Win7, the ability for extra-small (72dpi), small (96dpi), medium (120dpi), large (144dpi) and extra-large (192dpi) on the "simple" slider and the
Which means %/em/pt fonts under windows can and should be expected to be able to automatically change their size to fit the user preference anywhere from 12px to 32px -- despite the claims by some of the "experts" working on webkit claiming all browsers "default to 16px".
This is why it is the preferred approach to NEVER declare fonts in px -- px are a fixed size and should NOT be resized unless you resize EVERYTHING declared in PX (something FF thanks to it being Nyetscape 4's sweetly retarded cousin has always screwed up)... The only time there's any real justification to using PX is when you have a fixed height or really narrow fixed width that the layout would break at default zoom when the system metric is applied... which basically means an image interaction or button.
Which is why several years ago I made this page comparing the default sizes/metrics
Effect of Font Metrics on Common Sizes
Pay particular attention to the table about halfway down the page that shows medium/100%/16px/12pt side-by-side. I always get a good laugh out of how Gecko pays attention to the system metric for PT, but incorrectly ignores it on every other font scale.