Bill Gates' question made some sense. If mails are 3k each, and you have 1 GB of space, how many mails do you need to receive per day to get out of space in 6 months?
http://www.wolframalpha.com/input/?i=%281+gigabyte+%2F+3kilo...
the answer is 1826 mails per day. I assume what happened here is that they didn't have good stats of the mail sizes in the conversation and Bill tried to estimate something like this and as a result, he thought it was ridiculous.