I think that many people are just clicking "quick reply" when they reply to threads and the default font is small (like this).
I try to click the regular "reply" so that I can choose a font size, this one usually. My old eyes find this much easier to read. If I enlarge the entire screen to read the smaller font then the page doesn't fit nicely on my laptop.
First world problems, yeah. But I'm wondering if that default font that's in the "quick reply" box could be enlarged?