that would be the fault of the less experienced player, not of the weaker one.
damage = hp loss. it doesnt really mean that you lose stamina based on how much damage is done. you lose stamina and ki through your own actions, not your opponents.
and besides that, the higher leveled character gets the same loss of defense and attack as the lower leveled character.
at 100% stamina your defense is at 115%, and maybe varies with transformations. at 80% stamina your defense is at 90%. at 50% stamina your at 75% of your overall defense. 75 is the lowest it can go.
same goes for ki (unless this idea is disregarded). at 100% ki you have 105% attack. at 50% of your ki you have only 90% of your attack. and at its lowest, which is 20%, your attack is at 80% of its original. of course, only estimations.
just to summarize, the idea cant unbalance the game, in theory at least. it even makes sense that if you have less ki, you wont have as much energy in your punches. conversley it makes sense that if your tired, you wont be able to stand attacks all too well.