I speak of recent studies that show that the largest losses were in summer months, so the great army of Napoleon wasn't broken just by winter/distance/logistics.
Indeed, Napoléon's army was much depleted in its march from the Niemen to Moscow. But many of those losses were only temporary: many wounded, sick, stragglers, ... caught up with the army latter, when it finally stopped in and around Moscow. Other were left as garrisons in cities along the line of operation, or just left in hospitals to recover in those same cities, expending those garrisons when fit or marched to the army.
At Moscow, the Grande Armée actually recovered from some of its wounds, and all along the retrat, they also amalgamated garrison and depots in all the city they passed by. Near the Berezina, they even caught up with two army corps that had been detached on the flanks.
So, yes, the Grande Armée
had already suffered a lot from its march toward Moscow, maybe 50%, but all of them were not definitive and some caught up later with the Army, either at Moscow or during the retreat.
When marshall Davout made a roll call at the end of the campaign and established that his III. Corps (the most diciplined and well organized of the Grande Armée
) had lost 95% of its strenght, those were at least for half of them due to the retreat, both combat and attrition (lack of food, exhaustion, sickness, cold, ...).