A beginners question about levies

Hey guys. I just recently started playing CK2. After reading the sub's wiki I decided to play "tutorial Ireland" a few times. I got a bit better every try. One time I even had 3 provinces. (heh)

Now every time a war starts between me and another single independent province. His army is a lot bigger than mine. I upgraded the castle and made my martial train troops. But the enemy army was still bigger. How is this possible?

This is obviously a noob question, but I couldn't find an answer on the internet. So I'm so so sorry to bother you guys with it...