*Despite Allied bombardments German wartime production actually peaked during 1944
That statement is thrown around far too often.
While they failed the ultimate goal of destroying German morale and crippling the economy and the bombing campaigns were extremely costly for the Allies, it wasn't totally ineffective.
First, the Germans were late at dedicating their economy solely to the War effort (I believe it was 1944 before they fully converted), and secondly just because it was growing doesn't indicate what the potential growth could have been.
Second, the amount of the economy, military, and research efforts diverted to defending cities from Allied bombing was quite considerable. Every extra AA gun battery in Western Europe took potential men and materiel from the front lines.
So in essense a "Mega-City" in WWII terms would be a city of around 1 Million or More.
I would also look at industrial output. A city may have a slightly smaller population, but be a major centre of industry.
How DID they occupy cities in WWII? After defeating the enemy army in the field, did they just march down the street and that was it? Or did they also to have to destroy the local police force and militias?
Every city was different. Once Paris was directly threatened both sides surrendered it without a fight.
Other cases, such as Leningrad or Dunkirk (in 44/45) the cities were besieged and bombarded for long periods of time (neither example was defeated). The idea being to starve the enemy into surrender or (especially in the West) that assaulting had little value and would cost more than besieging.
Then you get the opposite end where every room of every building is contested, troops are sent in to defend and attack.
And every point in between these three on the spectrum.
Civilian, police, and militia involvement was just as varied (from the defense of the Danzig post office to German capture of Paris), but from my knowledge usually followed the military lead (i.e. battles like Stalingrad and Berlin that the military defended had many civilians join the fighting, whether by choice or not, while when the military declared a city open or simply abandoned it they usually didn't fight).