I think it largely depends on the nation, whether or not to expand the armies. Take China, it needs a large army so it's current regime can stay alive and repel a occupation. Guerilla warfare, in large numbers, will wear a enemy down eventually. Or take the US, it needs many troopers to expand it's "democratic empire" and/or respond to multiple threats plaguing the world. Or take something like Norway.... it just needs to fulfill it's duties as a member of the UN... so stay small.