There seems to be quite a few people around. There is an episode when Ed and Al are trying to lure Scar to them by performing lots of 'alchemy miracles' of all sorts, and you can see crowds gathering around them.
I just think tat Armestris is not densely populated on a whole; troops gather around the borders (together with a major part of the industry, etc)which is crucial to defending the country against the enemies, and then the rest of the population is spread across the country in tiny villages like Risembool. Central still seems quite lively.