I detailed why Russia was easier to take now. Because Germany can put larger stacks of units on the front lines with all those extra AA Guns. Russia will now have to armor to “trade” with the Germans. Perhaps not to the degree I detailed, but to some degree.
For instance:
Germany has 5 Infantry, Artillery, Armor, AA Gun in Baltic States, N. Ukraine and S. Ukraine. Russia only has 3 planes and no matter where they go, they are at risk of AA Gun Fire. Germany might be in the same boat, but can better afford lost planes.
Before, you only had 1 maybe you had a second AA Gun on the front lines, but you kept it back with your main force for later, when you wanted to stack next to the enemy. Now you have 6 guns, you can easily lose 5 guns to the enemy, especially if it means they have to send armored units into your range.