There are lots of themes on WordPress.com - different colours, layouts, flowers, no-flowers, serifs, no serifs, etc. There are none with a slightly bigger font.
Some of us write for readers.
That is, we write for readers who may not have 20/20 vision. We don't expect them to press their noses against their screens to read our text, and why should we? We aren't writing academic theses, we aren't blogging the constitution or tomes of legal gazettes, we're blasting out short tracts of text. A slightly bigger font can be afforded.
Yes, I know that I can buy a CSS upgrade and I know that most browsers support font-zooming, I just think it would be nice if a few of the default themes catered for people who like their sans-serif fonts to have something to hold on to, in bed.
This, my comrades, is the age of the widescreen.