After reviewing all of the Heartlands, Eastern Marches & South Coast, I have come to the following conclusions:
1. Generally speaking, most populated realms gain between 6-12 additional source levels.
2. More sparsely populated realms gain considerably less. Excluding the 1 province realms, Coeranys gains only 2 source levels (in the capital), Tournen gains 3 levels, Tornilien gains 2, etc.
This rule does seem to 'favor' larger realms ATM or rather it will somewhat favor their affliated magi (if they have one), since we are adding around the magnitude of 50-75 source levels overall to all of Anuire, the change isn't actually that material overall, but it will give Wizzies a little more bite and it could also benefit some of the temples that can wield sources.