Just because being cornered gives him the possibility to pump ATguns, grens, etc, like a damned factory while I struggle to replace my losses due to the stupid upkeep that penalizes the player who controls all the map.The upkeep penalizes the player with more units on the field, it has no connection to map control.
The penalty is not enough to be able to turn the games around on its own. It is the callins that are the problem here.
Let's say you have 50 pop more than your opponent. That is HUGE. Your army is DOUBLE his size at this point.
This translates into extra 75 manpower per minute for him, comparatively. Whoop dee do, every four minutes he can get an extra infantry squad compared to you. What is your huge army doing all this time? Sitting down? Extra MP is an advantage, of course, but as far as advantages go I would take having an army double his size every day of the week.
Now consider that this advantage could also translate into an extra Tiger for him compared to you, after 9 minutes or so. And THERE we have a problem.