Big or small team members in Heroclix?
From my experience, I would lean towards the "Go big or go home" school of thought on this. Here's why:


Most of the expensive figures have a higher defense value. Most of the less expensive figures have a lower attack value. You may be able to direct 2 or 3 figures towards attacking your opponents costly Superman or Hulk, but they will have a harder time hitting it. It also works the other way: the higher cost figures will be able to hit your figures more often. Which leads me to...
Damage. The same idea applies for damage output, hit points, and defensive abilities. Your lower cost figures may have great stats on the first click, but as soon as they take any damage, their stats will degrade very quickly. The stronger figures will do more damage, making your figures less effective than you have bargained for. (This is especially true if your figures get hit before they have a change to attack.)
The higher cost figures tend to have higher damage output as well. If an expensive figure is doing 3 or 4 damage with a single hit, most 50 point characters will not last long.
Lower damage output also makes a costly figure's defensive bonuses better. If they are Tough or Invulnerable and you're only doing a couple of points of damage which each figure, you're not going to make much of a dent.

Swarming seems like a good strategy, but most of the time a higher cost figure will have enhanced defense or defensive abilities that keep you from doing a lot of damage to it and removing it from the game.