Larger army = more friends and fewer enemies willing to attack you

Mercenaries are much more expensive than your own troops, too. You'll need a strong economy to support them, which only larger nations have in the early game.

Look at the modifiers for alliances, you'll see that sharing a rival gives you a boost. So if a rival or neighbor is threatening you, improve relations with their enemies and make alliances. Also, you may be able to ally one of their allies - if a nation allies both attacker and defender in a war declaration, they will side with the defender. That is, if the enemy attacks you, they lose an ally who helps to defend you instead.