I would look not at the campaining but at the results, the harvest. Harvest can be good or bad. When too many people are necessary for the army, the harvest will be bad.
When the harvest is bad, other things will be bad, like the manpower of the province, the morale of the people, the attrition of the armies, the growth of the province, etc. When the harvest is good, manpower and others will be better.
A leader of a province can decide to use many people as soldiers, but in that case he will see that the value of his province goes down. If he can keep his people at work and not use them as soldiers, the value of his province will go up. When speaking in EU-terms I would say that the base-tax of that province can go up and down, simulating whether people have enough food or are hungry. And when base-tax goes up and down, things like manpower, growth, attrition in that province, etc should also go up and down.