More overpop means faster troop desertions. When your def starts to drop very fast, people can easily make multitaps against you to drop your land even faster. I play in a kingdom that uses deep chains. We drop provinces below 300 nearly every war. Based on my experience I'm telling you that mods from buildings are better than mods from more pop.
We're currently at war, but we're between 1.5 and 2 mil netw and I'd be happy to war your kingdom and show you.