Things are designed such that they explicitly conceal the true size of Garlemald's forces. Garlean tactics tend to centre on crack-squads rather than presenting a large standing army.
We also have the size of the planet to consider. By Earth standards, what we see in Aldenard would suggest it isn't much larger than the Brittish Isles. Small planet.

And on that planet, people can cast return. Now imagine how strong armies are and what tactics they can employ with this skill at their disposal. Having actual respawn points at your home base changes everything.