Ceric: It isn't becoming lighter per say. Its simply the amount of Damage received by a character determines the distance he will be pushed back by attacks. He isn't becoming lighter.
The biggest problem with actually making characters become lighter is that fighting games is about timing, and if you have learned Bowsers timing for his special moves, Smash attacks, and generally getting around then you are always expecting that reaction time and planning to use it to your advantage. If that timing changes mid-game because someone has attacked you to the damage level that it makes you lighter. You may have an advantage, but it could also be a disadvantage because it throw your timing off.
As for smaller characters never being able to throw larger characters being unfair. I think you must remember to take in to consideration all the other advantageous smaller characters have. Very quick attacks that can help them hit and rack up damage against a heavy character and fade away. Usually, larger jumps and better triple jumps. This one aspect of throws is a minor aspect of the game, and it could be programmed that smaller characters just do something different than a throw to larger characters.
Also remember that I have been discussing fundamentally changing characters like Bowser to where they don't even have a triple jump, so that would also balance out the no throws, and the heavier Smash attacks they recieve.
Bill: I am not per say trying to make it more complicated. I am trying to add character balance, and a means to still create creative and original fighters for the game. As for my inclusion of Air Dashs, and a Combo counter, I think both of those are long over due.