Menchi had mentioned this point on Wikitech-l:
"Chinese characters have many many more strokes than Roman letters. So the current MonoBook font size at Chinese WP is very hard on the eye."
"Almost every single Chinese user who experienced the skin have pointed out that the font size's difficult to read."
So it is not a problem for one user's preference, but a problem for the site.
Please change the default size to 11pt which is adopted by most portals.