All right, it's definitely far too hard to earn experience in non auto resolve battles.

I fight one battle. I am outnumbered badly. I utterly crush the opponents forces, slaying all but 4 men.

Not a single unit of mine gains experience. Not the hastati that lost 4 men and killed hundreds, not the hastati that lost 80% of their strength and killed hundreds.

I reload.

I auto resolve the same battle.

It's a 'CLEAR DEFEAT'.

Oh but wait! THREE QUARTERS of my army gained an experience level.