That is a good point, but the difference between a heavy and a light is more than health. It's still a relatively fair trade-off, since the amount of time it will take for a heavy to get where a light can get, heavies may come to not even bother trying, altogether.
I think, actually, that this is pretty fair, now that lights get the same about of ability pips as heavies. Why not let the heavies go to the same places, but a little slower.
I'm just thinking that "fair" isn't being assessed properly. Why not let the lights carry the same guns as the heavys, but a little slower?
From a strictly tactical standpoint, there appeared to be two tradeoffs made going between light and heavy, with the Medium balanced between them.
Armor vs Speed and Firepower vs Mobility
Now, if it's a matter of the heavies having to walk up 6 flights of stairs to get to the place that the light scurried up the walls to, maybe that doesn't change that balance (but we have to see maps to know that), but if it's that the heavies can climb up the wall too I hesitate to believe that the lightweights maintain a certan amount of their usefulness.