Specifically, the font sizes within the AdSense interface, where Google AdSense publishers maintain their ads, view reports and make tweaks to the ads and targeting, has all shrunk.
One publisher said:
For the past 2 days, the fonts on the AdSense Interface have been ridiculously small. At first, I thought I somehow mucked up my browser settings, but it's the same on other computers I use.
Is the AdSense Team trying to push people to the new interface by slowly shrinking the fonts to minuscule size?
I personally cannot replicate it on my browser, but it seems to be affecting some users. So if things look really small, don't blame your eye sight - it is likely a bug.
Forum discussion at WebmasterWorld.