Yes, indeed. It's to keep the balance intact; since the monsters are made as not too easy/not too difficult, when you handle them on your own, the fighting would become less hard. Therefor the monsters become stronger to neutralize the increasing in numbers of the hero's. So if you're looking for a little more challenge, join a server that has already like 5 players, and try to fight on your own (or not, fighting in a group can also be fun)
Decide what you want, decide what you are willing to exchange for it. Establish your priorities and go to work. - H.L. Hunt