It happens for me, too, in Mozilla 1.7.7.
It appears that everywhere in the discussion fora except Bugs & Comments, the gunks_sm.css style sheet is used. However, B&C uses blind.css. An extremely cursory comparison of the two shows differences in font size specifications (and an unfortunate and confusing mixing of percentage and absolute pixel sizing).
Of course it's easily adjusted with the browser's font size increase/decrease controls. (ctrl+/ctrl- in Moz, ctrl-scroll wheel in IE, or via browser menu in both)