Yes I agree that a double food would be a better function.

BUT, I think this could all be avoided/counteracted by a Trade Balance adjusted for chained province.
At end of war, I think EVERYONE should have thier TB adjusted (Actual accumulated balance) based upon KD average acres (Net might skew too low after war)

Wiki says:

Aid to needy provinces is sometimes exempt from taxation. If the receiving province has been hurt significantly in recent days, aid may face a reduced tax rate (down to a minimum of 15%). This aid still accumulates in the Trade Balance of both parties. Also, if the province is below the average networth of the Kingdom, then the tax rate is calculated on the average kingdom province networth.

I think that TB (% taxed) adjusted by KD Average Net should be impleneted OUTSIDE of war.
After a war (end of war cease fire) TB (Actual accumulated balance)should be adjusted to according to KD Average Acres.

Yes this also means that those higher than the average will accrue a negative TB.
I think that this would encourage the bigger provs to aid the smaller provs.