I generally find that the lower-level events tend to be full - anything E-B is best, definitely if they're spec series events. Longer races tend to be emptier, especially endurance races, which is a shame as I love them.
Also, don't skip to the next race from the post race menu after finishing one unless the lobby you've just finished is full as you'll find that you carry on with the same group of players when you move to the next race - instead, back out and choose the second event - yes, you'll have to wait longer for the next race, but you have more time for the lobby to fill up.