I´m mainly using SnowLeopard, mostly Safari and Chrome over the last weeks, so I noticed no difference in the fontsize of xenforo so far.
When I was on Windows 7 yesterday and opened xenforo on Chrome for the first time, I noticed the significantly smaller size of the message-font.
So I performed a comparison on both operating systems with the following results:
Firefox 3.6, Chrome 6 and Safari (most recent version) and Opera 10 are having the same look exactly (smaller fonts).
Chrome 6, Safari 5 and Opera 10 are having the same look - with larger fonts than their windows-versions. Only Firefox 3.6 is giving the same impression like it´s windows-counterpart with a smaller font-size.
I like the smaller font-size more, but I have no clue how to force Chrome, Safari and Opera on Snow Leopard to give me this look.
Maybe a noob-question, but I couldn´t resist.