I think a large part of it is that the only things we can upload here are text and a profile picture. Raw text data takes up very little storage space, and with sufficient compression you can keep the file size of profile pictures relatively small as well.
For example, my profile picture is ~60kb. If we assume the average user uses about 10mb (1-2mb picture, 8-9mb text content), that's 1tb storage per 100,000 users, which isn't a terrible amount. With a decent compression system for blogs/forum topics that haven't been viewed in over a month, that number could likely be brought down to half that.
I'm not saying that trying to run the site on stickers is a sustainable model, but it also isn't worth worrying too much over.
Personally, if An were to ever offer something like different profile templates or an in-browser profile editor for like a dollar I'd probably spend some money on that. There are other ways to fund the site than ads, if it comes to it.