Yeah, but so what. If you war against these giant stacks (I've never seen them THAT big...), let them walk on your land, and let attrition do its thing. Then you can easily kill the armies. The thing is, the province support limit is affected by many things - technology and ownership is one thing, I believe ownership is x3. So while you are fine standing with 80K troops in your own territory, the equivalent territory belonging to your enemy is only one third. This also makes it a lot harder to be aggresive with huge stacks, though!