Not sure if it's still true today but a while ago, the number of bot accounts actually outnumbered the genuine accounts:
http://bitcointalk.org/index.php?topic=934827And having so many useless accounts on a forum can in fact make the database large. Not sure about SMF but back when I managed a phpBB board, it was definitely the case.
Most of that discussion centers around spambot accounts but farmed accounts for selling are almost certainly a part of this mix.