Don't most browsers remember your zoom when you revisit a website?
Yes, but why have such an hard to read default?
I'm not sure but it could be that it makes it easier to read on big displays. Of course this isn't very Responsive Design like. I will be glad to be proven wrong. Interested in why HN has such a small font by default as well.
Maybe it's a mobile issue? I've never adjusted zoom in my phone.
I was going to suggest bump up the saturation/blackness of the font so that it's easier to read.