I really like that PDX researched the historic manpower levels and put them into their database, but...
this lead to complete unrealistic available manpower with ridiculous number of divisions available to all powers after some time (e.g. a vanilla and 1936 sized Hungary able to field more than 70 divisions, etc.).
My suggestion:
- Keep the manpower levels, but add an agricultural tech level (like it was available in the former HOI versions). This will factor the available manpower according to the advance of the local agricultural output. This will especially reduce the available manpower of underdeveloped countries like India or China, because they would need to devout a greater slice of their pop to feeding instead of fighting compared to more developed countries. You can overcome this negative factor by advancing in according techs (e.g. tractors, farming machinery, land reform, fertilizer, etc.), or by rationing food (but this should lead to decreased NU after a while [see historic effects to the central powers in WWI]) or by exploiting conquered territory (but this should increase local resistance).
- Historic facts: Improving the agricultural output between WWI and WWII was one of the reasons which allowed the German Army to increase the available manpower to unheard levels.
- Introduce the national military administration as another factor that will drain available manpower. All powers had to use considerably administration overhead in order to organize, train, supply and move their armies. In order to add spice to this negative factor, each country should be able to tweak their military administration to serve their needs. Specializing into one of these areas should reduce the available manpower by an extra factor. Historic examples:
- Germany could focus on an Ersatzheer system, which allows them to preserve unit XP from combat casualties, due to their complex R&R system. It took lengths to ensure that wounded service men returned to their former units and not to an anonymous replacement pool. Each division also had a replacement unit where new recruits were trained as a unit by their future NCOs and officers prior to their assignment to the front.This increased the moral of the replacement and the effectiveness of the refitted unit.
- USSR could focus on a highly advanced reserve system, which decreases training time for new divisions during wartime. This would allow them to increase their army size quickly and efficiently in times of need.
- USA could focus on a complex supply system which allows them to supply their troops fighting abroad via two oceans.
All in all these negative factors to the available manpower should be heavy enough to allow the historic division numbers to be fielded. But if you would want to increase these numbers, you need to make meaningful decisions with real consequences.
These are just me 2 cents.