Quite recently the Google AdSense team rolled out new changes that allowed you to control the font sizes and fonts for the ads, however by default Google started applying medium font sizes by default.
In another announcement today the AdSense team has decided to change the default font face that will be used for displaying ads, currently most of the ads are displayed using Arial and Verdana, the new default font faces for ads will be as follows;
Arial: 728×90, 336×280, 120×600, 120×240
Verdana: 300×250, 160×600, 468×60, 250×250, 234×60, 125×125, 180×150
Times New Roman: 200×200
If you are not happy with the default font face you can change it by editing the ads.
Default font faces to be updated [Official Google AdSense Blog]