The "badge details" pages are showing lists of posts that awarded the badge in different font sizes:
This is affecting only some of the badges. Others use the same size for all posts on their page, but different size from other pages: for example, Nice Question shows all posts in the same font size (the larger one from the screenshot) while Altruist shows all posts in the smaller size.
At @ChrisF suggestion, I checked if questions and answers appeared in different sizes and that seems to be the case: questions are in the larger font and answers are in the smaller.
Browser/OS: Chrome 13.0.782.112 on Windows 7 64-bit.