Hey everyone. So, as an attribute to my "dedication" I'm trying to decide and formulate the population sizes of my various countries for my fantasy world. While I'm not trying to be TOO detailed, I seem to have hit a snag. Researching a little classical demography, there were easily upwards of 20 million people living in just Europe around the middle ages, if not more. The classical roman empire like-wise just had 30 mil just in Europe, with another 20 elsewhere.

However, the problem is that for my fantasy setting, these numbers SEEM very large, even considering the rough equal land mass. But, how do I reconcile a large population of humans, or huminoids with areas that should also be populated with Dragons and other monsters ready to EAT and or other dispicible things to them. The sources I have for my games put cities (the largest unit save unique locals) at around 25,000 per, but how packed should these cities be to each other?
In short: For those who care about these types of things - would it seem more believable to lean towards more historical accurate populations sizes, and if not, how much should they be reduced to be believable within a fantasy setting?