viral - 1:05 am on Jul 1, 2013 (gmt 0)
I am pretty sure Matt Cutts even mentions readability in a recent video. He doesn't talk about font size as I recall, but he does talk about readability and I assume font size would be a part of that.
As Hoople says above it may be an indirect consequence of bounce rate dropping.
You should go back through your analytics logs and see if you where ever hit by the Panda update. There could be a correlation between font size and panda. Something I have not considered before.