I think simply put, larger monsters are better and put out more damage - thus we get fewer minis per group. They are great all around tanks. Smaller minions are great early on as they can drown heroes in numbers of attacks and possible damage. FFG playtests for balance, and I believe the balance in 2nd edition is pretty great.
However, from the overlord's POV, some heroes have some fancy gear and can also be tough to crack (especially later in campaigns). I think that the overlord needs some powerful monsters to keep them afloat and make things more interesting.