In Medieval, you could really thrash the enemy while outnumbered if you use the proper counters.
That's one thing Medieval has a leg up on over Empire. In the medieval period, there were proper counters. In Empire era, the majority of your units use muskets. So your counters for muskets are.. more and better muskets. There are of course some counters. Pikemen still exist and some cavalry still exists and pikemen beat cavalry, anything else beats pikemen, and cavalry might beat everything else, as long as you dont' do a frontal charge. There are also cannons, but most of their ammo types are rather flimsy and uninteresting for dealing with troops. The best shot type they have (scatter shot, short range but it rips through groups of infantry) is also the most low tech so it gets forcibly replaced as you research new technologies.
It's still possible to win outnumbered matches in E:TW, but it relies more on having higher tech units than proper counters 