It's absolutely arbitrary, but it's a small enough number that everyone has a chance to contribute to the discussion. With this particular class it had nothing

It's absolutely arbitrary, but it's a small enough number that everyone has a chance to contribute to the discussion. With this particular class it had nothing to do with resource constraints... maybe more to do with how many students were interested. I didn't have a single anthropology class after 101 that had more than 35 people in it (and this was at a large state school).