There's another reason too, and that has to do with my interest levels in books. When I read, I like to feel like I've accomplished something in a half hour of reading. I'm not a fast reader, so when I spend time reading, it's nice to know that I've gone farther than five pages in a ten minute span of time. If it takes me an hour to get through a relatively small chapter, then I start losing interest in the reading. There is a book I currently have on my review shelf that suffers from this, and the result is that I'm no longer reading it. I might try again in the future, but for now, I can't be bothered with it. It's a 6x9 trade paperback with font this size. Or maybe smaller. I don't now. How big does that little bit look on the screen?
On the other side of things, though, there are books with font size that is too big. For children's books, this is perfectly acceptable, since kids really can't be bothered to read normal-people font anyway. For adult books, however, large font is kind of cumbersome. I don't think I've ever dropped a book that had large font, but it can still make you a little irritated when you bought a 300-page novel only to find out that it's actually 150 in more traditional font sizes.
So, to anyone publishing books out there, please use a reasonable font size. I know you want to save paper and all, but what is more important: a book that becomes a nice door stop, or a book that gets read all the way through and enjoyed for what it is, rather than hated for how it was put together?
What other pet peeves do you all have about the design of books? Let me know in the comments!