In every single lore account, Demacia has outnumbered Noxus. Last update on this with Sion's update was that a single battle saw Demacia outnumber Noxus ten to one.
The only possible way this could be the case with how big both countries are - that one would outnumber the other so substantially - would be that Demacia runs a military draft. Noxus' army consists of trained, elite warriors because theirs is a volunteer army and is filled with people who actually want to fight for their country.
Even facing destruction from an enemy as massive as Demacia, Noxus continues to hold out and maintain a volunteer army versus forcing people to fight.
I don't know if this was intentional, but I enjoy this kind of moral ambiguity. Having any one side be a saint and the other the goddamn devil is what children's tales are made of, not good storytelling.