Fonts in Adsense

I know that you can't change font sizes in Adsense but on some Adsense ads like on this forum the fonts are smaller and Verdana while on my site they aren't. Am I missing something or is there a way you can specify what font size the Adsense ads can have and maybe what font to use as well? I haven't been able to reproduce it on my site whatever I try.

I have not tried changing the font size here. However it may be a side effect of some of the css I use.

I agree with Chris,

It's probably the CSS that is causing it to change. I think it has happened to my Adsense ads know and again when I have been messing around with my CSS file.