"I remember getting in a room to plan our RocketMail launch over a decade ago and worrying that our original plan of a 2MB quota wasn’t enough, and that we needed to be radical and DOUBLE the storage to 4MB per account," said Yahoo! Mail's John Kremer. "It’s ironic that I routinely send and receive individual mail attachments bigger than that now. Our total capacity for mail accounts back then was 200GB for all of our customers. At Yahoo!, we’re now receiving more inbound mail than that every 10 minutes."
However, they have more accounts than any other provider today. That space can't be cheap, for sure. Unlimited never is, but it's quite possible they may offer more than 99% of people could ever use. At least, in today's terms. Five years from now, 2GB might be considered a bare minimum.