Take for example this match I had tonight. On one team you have 5 mediums, 2 lights and a single heavy for I think 355 tons total. The other team has 3 assault mechs (one a hero), 3 heavy mechs (one a hero again) plus a medium and light to round things out at about 565 tons. So this is basically a group of mediums pitted against a group of heavies and assaults.
This kind of imbalanced match leads to teams getting slaughtered, their pilots pulled down by all that extra weight while their trimmer, fitter foes crush them with ease. This match saw the enemy team move to capture theta while their one light capture kappa. They then regrouped inbetween and where attacked by most of the other team in a grand melee that saw their fat, stubby mechs brutally murdered with no chance to defend themselves.

Dropping teams with few or no mediums or lights to defend themselves with is like a death sentence. The match maker needs to ensure that at least a few good mechs are included to balance out all the big wastes of metal.