"Medieval" on its own is a very broad time period. Your question literally spans thousands of years. You might have better luck getting a good answer if you specify a (much) more specific time period and a place (e.g. Northern Europe in the 8th century, or something like that).

Most of the time, they didn't. throughout most of the medieval period, especially in the earlier parts, armies were absolutely tiny compared to what Rome could field. This was because the individual states were smaller and also because they were more decentralized (ruler had less power) so the logistics for larger armies just wasn't there. As time wore on, states got larger and better organized, and armies started to grow, but it wasn't until the early modern era when huge armies became common again. There are tons of exceptions to this, but it was more of a general trend than a rule.