We're never given hard army numbers. The fey are global powers for a reason and it isn't because everyone else is too nice to want their seat.because full blown war is costly and its estimated winters standing forces are like 40k? Losing thousands of soldiers when the white council is already at war with them isn't the best bet when they can just wait first.
Also you know mab clearly somewhat masterminded the events in canon to end up in their extinction. Not completely of course but it was definitely a factor in their complete destruction.
They were also politically on fire for a lot of the war because Nemesis burned centuries of resources to mess with them.The Fae are immortal, and take a long view of grudges.
That doesnt mean they dont settle them.
It had one of the Summer queens in thrall, and even under normal circumstances Summer is supposed to keep Winter from playing internal security.
I don't think it's coincidence that the end of that plot resulted in the fey beginning to move on the Reds.