yeah, it's just counting it like this:
the most recent post numbers are around 7,200,000. And since there's deleted posts, the "reasons" must still hang around, so we should keep them in the Post Average.
Let's say all posts were exactly 25 characters. You would get around 1.7 GB.
Now you average it out to a more realistic number, say, 100 characters? That's where I got 17 GB, and rounded up to 20 GB for a more accurate estimate (because here people are good with the "constructive posting" part of that).
And if they used the UTF-32 (which is a lot more than UTF-8 and such)...damn.