I know this might sound bad but leaf get easiest missions.
-They got waterfall insdie villige
-Firest full of herbs
-Animals all around
This means that they got easy missions -> More Influence gain -> GA -> Fast trained shinobi
I think you shuld make a mission like get a Pinch of Sand and chop a bad tree ( The ones without leaves near sound villige) to make missions more fair for all villiges.