When an Empire is a over 200 realm size, occupying a mere 12 counties, just over 2% of the relevant territory, should not be sufficient to win the war outright, regardless of battles or holding the objective. But that is currently the case, at least with popular uprisings.
This seems highly unintuitive and possibly unintended, so I flagged it as a bug, but even if it's working as intended, it should absolutely be changed. It makes absolutely no sense that a ruler would feel compelled to submit to their demands under such a circumstance, and the peasants were absolutely not in a position to actually enforce demands.
I'd suggest that similarly to how warscore from battles is scaled to the size of the army, warscore from occupation should scale to the size of the realm. It makes sense that some holdings like the ruler's demesne or capital would be worth more, but it makes no sense that briefly holding a few castles and on the outskirts of a large empire could be used to demand control over the majority of that empire's land.