ORIGINAL: Hubert Cater
The USSR can stay afloat for quite some time via the MPPs they maintain after the Industry transfer to the Urals as well as the arctic convoy help they receive from the US/UK. That and they are also able to repurchase many destroyed units at 60% of their original cost which helps.
But that being said I suspect the current bug, Soviet Corps/Army stats bug, is the biggest factor that is working against you in this game. This is currently helping them to maintain their combat strengths more effectively than they should, at least in the initial war years, and they are likely not losing as many units as they should under normal circumstances. For example, by the end of 1942 it is probably not a stretch to have seen them lose well over a 100 units when looking at the losses charts. If it is less than that in your game, and if they are not being pushed back and losing enough MPP collection resources such as cities/towns and mines etc., then this is probably having a bit of a snowball effect.
The good news is that the patch should be ready relatively soon, likely early next week if all goes well and you'll just have to start a new game from scratch to see an effective difference in your game.
Curious is this affecting WAW also or just WIE?
Just WIE, as having different base unit stats by nationality isn't used in WAW.