Let's be honest... normally the biggest army wins.It's as simple as that. Especially in a simulation, however adding elements which can overrule the standard rule of thumb, makes things more interesting.CK2's combat system sounds good, but in practice it included so many noise factors that battles boiled down to whoever had more dudes would win in the overwhelming majority of cases.
You need quite some factors stacked in your favor to normally win battles with huge differences. History isn't all 300 Spartans..(Actually in RL the Spartans lost that fight..)