Watched this again - 1 question

Watched this on a lousy DVD copy a few times when I was young, today watched it online 15 years later. Still enjoyed it.

But I don't get why Aladar is such a weakling compared to the two brothers? Aren't they all the same type of dinosaur, and isn't Aladar supposed to be a fully grown adult? Why do the brothers look more gruesome and stronger?