Though it depends on the era. Cannons made old style fortresses obsolete. Before that they were very hard to capture. This problem usually led to a king prefering to negotiate with local strongmen who held the castles. Resulting in feudalism.
It's not by chance that the strong central governments emerged after gunpowder weapons became mature. What I'm trying to say is that even in the real world it was very hard to capture castles. Siege weapons before gunpowder were ineffectual and usually it came down to starving your opponent.
OTOH fantasy does have it's offensive perks, like trolls or dragons. But you usually have something to negate them with.
Still, I can think of some rare examples of the bigger army winning.
Have you seen the original Heavy Metal movie? The evil horde of the green stone easily sacks a city.
You have the fight over the oil well in Road Warrior. Though it's sort of a tie.
I think Rand's Aiel had the number advantage on the Shaido in WoT.
The moredhel broke the defense of Armengar in A Darkness in Sethanon.