(Reposted since I forgot a question mark in the title last time)

Walter Scheidel explains in his “Escape from Rome” about how Rome coudl raise frankly mind boggling numbers of soldiers, like, more than would be mobilized against until like the 17th century in Europe, give or take.

It seems like a majority of troops were raised from its core Italian territory, before the expansion of the empire, many of these huge levies were raised before they could even draw troops from its colonies.

Even assuming the extraordinary levels of mobilization they achieved, this still seems to assume a pretty massive population in Italy, far large, as a proportion of the Europe/Mediterranean world than it is now or even for most of the last millennia or two?

How did it have so many people? I have identified a couple possibilities but I could be missing something?

1. Does it have to do with the “Roman warm period?” If so, how much of it was this?
2. Are my numbers wrong as a proportion and it was just their huge level of mobilization that makes things so impressive?
3. Other stuff I’m not considering?