Your two dimensional posing of the question goes some way to explaining why you are not succeeding in a complex game like WitE.
Let's assume you successfully retreat to your non-attrition defence line without major loss. I follow you to a line of my choice and dig in. We both refit over the winter and you are then confronted by a defensive wall 3 or 4 hexes deep of level 3, 4, even level 5 forts. You will probably spend most of the summer trying to break through the line as I increase in experience, morale, leadership and combat capabilities of all sorts. You will have no first turn bonus, I will have no railcap penalty. The best you could possibly achieve is what you should have gained in 1941, but realisticly, you won't because the entire co-relation of forces has changed.
You will probably run out of oil as a couple of well placed strikes on Ploesti and Bucharest with 500 odd Pe8 and IL4 will have crippled your oil production.
Alternatively, you attempt to hold in Russia, and organise a fighting withdrawal when the blizzard starts, to a defencible position. You will suffer serious losses, perhaps crippling, depending on the skill of your defence. You may well end up knocked out of Russia anyway, but given an adequate defence, I will be weakened as well as you by the winter fighting. I will have had less time to dig defences and you probably will have less distance to advance before reaching important objectives.
Even if you hold in Russia you're batting on a very sticky wicket. But to withdraw shows me that the monster strike I thought might be deliverered from an unexpected direction by the experienced player you implicitly made yourself out to be, is just not going to come.
Clearly, the relationship between my additional manpower and the manpower, actually, both of us have lost, not just you, in your defending within Russia, is only a small part of the equation you sneeded to have considered.
“The end of democracy and the defeat of the American Revolution will occur when government falls into the hands of lending institutions and moneyed incorporations.”
¯ Thomas Jefferson