Thanks, I just finished this campaign - lost as the Allies in 1916, down to the last man basically. I compared research with my opponent and we were on a par except for his getting Infantry Weapons 3.
I think what actually created the effect I witnessed was my opponent's force management. He suggested that he was essentially resting a line of his divisions at all times, so I think that over the course of the game this meant that, combined with my higher rate of replacing divisions, his morale and efficiency levels were maxed out on most units, whereas mine were at or below 50%.
This makes some sense, and I'm seeing some success doing this in a new game. However I'll offer my logic which led to the early collapse of Russia: my plan was to wear my opponent down by consistently attacking along the line as long as the odds were not more than 1:2, with the thought that my numbers advantage and front advantage would lead to a bogged down German army.
I haven't checked if this is the case already, but it might be a nice touch that morale/efficiency can degrade even when defending or attacking successfully. Honestly the tempo of my last few games has felt like there was in fact no trench warfare at all, and the best PBEM players I have encountered seem to be in the habit of force concentration on one front and conducting a blitzkrieg. Surely the troops need to rest, even when they succeed in their mission :)