The tank warfare went from light and small gunned tanks to behemoths that could barely function. The U.S. chose to avoid the constant changing of tanks to concentrate on production, despite the costs of not having a decent heavy until the Pershing hit the shores.

What if the Germans had concentrated on smaller, faster tanks. Using them for fast attacks, with larger SP howitzers acting to destroy enemy tanks from long range. Additional support would be required, but the German forces could potentially be more effective and better able to handle Russian terrain.

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Lord, let me be the person my dog thinks I am.

German tanks became heavier and heavier because they could not develop ammunition to keep pace with armor thickness. So the only choice was to increase the main gun calibre. Bigger gun requires more room to mount ,store ammo and operate the gun, thus leading to a bigger tank. Bigger tank requires bigger engines further increasing internal under armored volume and therefore mass. To counter heavier enemy guns and ammo the germans had to increase tank weight even more to put thicker armor over larger tank volume/surface area. The thicker armor can then be seen to offset the lack of effective gun penetration.

Had they been able to develope an alternative to the Tungsten based HVAP shot, they could have kept the Panzer III going as the main battle tank into mid war period and supplmented that with StuG-III mounting 75mm gun, also firing the improved ammo. This way a more orderly and controlled transition to Panzer III replacment could be followed mounting the longer version of the 75mm gun.