Why is it that the romans were able to consistently raise armies of 100000 soldiers and a good proportion of them professional ones, and then in medieval times you've got a "big" army if you have 15000 men?

I maybe wrong and it's only this impression I get, but it seems as if there was a massive change in scale in everything from the size of states to the size of the armies. Now I know that a smaller state has fewer men and therefore less manpower and resources. But what caused the roman model to become outdated? I know that the Frankish empire was rather big at a time, but not for too long. Was it that the romans had a huge technological advantage with the areas they conquered and that's why they managed to stay in power? This seems rather dubious as the east was also advanced, wasn't it? And it stayed under the empire's grasp.

I guess I'd like to know why was it that for a long period in between big empires and nations did (western) Europe become so decentralized and "weak", or if it's a false impression I've been getting.