As for the number of enemy troops; the AI cheats a lot on support limits in order to field excess number of troops as minors. If you don't want the AI to cheat as much, play Very Easy. It will make you see how vulernable the AI really is and why it needs those cheats.
A side note on combatting minors: If the enemies forces are larger than yours, just try to get them into your territory to die of attrition if that's a possibility. Also try to make them attack you across rivers into mountains, swamps, and forests (plains and deserts if you have cavalry advantage). Make sure to cover their only province as well (this is the only tip that specifically only works against small nations, the others apply to majors as well).
A side note on combatting minors: If the enemies forces are larger than yours, just try to get them into your territory to die of attrition if that's a possibility. Also try to make them attack you across rivers into mountains, swamps, and forests (plains and deserts if you have cavalry advantage). Make sure to cover their only province as well (this is the only tip that specifically only works against small nations, the others apply to majors as well).