I am not sure if there is a penalty to the bombing if you overstack air. However, the more planes you bring, the more chances of flak taking out some of the units, and I notice a fair number of times, my fighters get hit by flak.
So I try to 'escort' by sweeping. I hit areas behind the lines with my fighters only, to try and 'use up' the Russian fighters. If he has too many fighters to use up, then I don't bomb in that area, and I transfer more fighters to deal with the problem.
I find the only way I can blow bridges over the major rivers, is to use around 200-300 stacking points of bombers. So more is better.
“My logisticians are a humorless lot … they know if my campaign fails, they are the first ones I will slay.” – Alexander the Great