For some historical reasons X11 sets its internal display resolution to 75 dpi. AFAIK all current Mac computers use displays with a far more better resolution than 75dpi. E.g. my 21'' iMac's display has 96 dpi.
Now, if we choose to set fonts in X11 applications, like Gimp, this will have some unwanted side effects. Mac OS X system font is Lucida Grande 10. If we set this font in Gimp.app's gtkrc it will really look tiny. Then why don't set the font size in gtkrc to something like 12 or 13? This will look nearly as tall as OS X own font size. But what if you put Gimp.app on a MacBook? My MacBook has a display resoultion of 113 dpi. So this will result again in tiny fonts. Now, what shall we do? Simply change the resolution X11 uses internally.
I found an article on macoshints.com about changing display resolution in X11.
http://www.macosxhints.com/article.php? ... 1201901244
This way it's possible to set the font in Gimp.app's gtkrc to 'LucidaGrande 10'. Combined with the correct dpi setting for your display panel, the font size Gimp uses should look like the OS X system font.
This topic was discussed on the XQuartz mailing list too. I don't know if they changed the way X11 detects the correct dpi, but it will do no harm if you modify your 'startx' on your own.