When Bungie showed off how much data Halo Reach was producing during it's first few months, I believe it was at the 6TB mark. Halo is a shooter, far less data than an MMORPG (but still a lot of data), yet it still produced that much data for not having all the bells and whistles of an MMORPG.

If you're comparing an eCommerce database with an online game database, that's where you're getting confused on the "high number" and why you think it's not possible to be high.