An International Spam Days

[ZiNgA BuRgA]

They're probably the automated bot posters.  Interestingly, human spammers seem to only target bigger boards.
Bigger boards will probably be moderated more so spam is unlikely to last long, but on the other hand, Google tends to crawl bigger boards more frequently, so it somewhat counteracts it.  Bigger boards will also have more posts, thus, perhaps a higher chance of a spam thread slipping through - and when one does, it's a fair bit more beneficial than spam on a smaller board.  So delegating smarter resources onto bigger boards make sense.
As for smaller boards, there's a higher chance that a spammer finds one unmoderated or rarely moderated, so probably more targeted by bots.

From my user tracking, I've found that most bots find your site via Googling "powered by mybb" or "inurl:showthread.php", so you could probably reduce the number of bots simply by:
1) Removing MyBB copyright or replacing it with an image
2) Customising the URLs a bit (either via SEO plugin or editing MyBB's SEF URLs)

Though I'm not sure how well the above measures would work on more established forums due to Google's caching.