As you heel, your are decreasing the cross sectional area of the sail to the wind, but increasing things like your tramp area to the wind (hence mesh tramps). You are also decreasing the amount of weight that you have that is put towards righting the boat. Since your weight stays normal to the ground line as you heel, instead of trying to right the boat, you are now just pushing down (varys with angle of heel) on it. The more you heel, the less you are doing towards righting the boat.