probably splats. that was an issue in dow2 as well.
I think it's a large part of the issue, but not the whole. However if you look in the world builder lots of the in game textures are actually made from splats and splines leading to huge performance issues, whereas on DoW II you could have 8 different painted textures, in this I believe you can only use 1, correct me if I'm wrong. This means if you want road surfaces or paths you have to use splines, so maps have excessive numbers of these in CoH 2.