Some guess
a. They mobilized
b. Not only do they mobilize, but because their score rose they can get more materials to build more troops
They mobilized but so did I. Since their population is close to mine, I don't understand how they ended up having more troops than me and UK combined. I also didn't attack them in their territory to minimize attrition damage. Simply put, we were comfortably outnumbered, when I or UK would destroy a stack, another would come up, there was no time to recuperate and I wasn't able to defend all parts of my territory, that's how much stacks they had.