Made some adjustments here and there

Among other things Mazovia has been reduced (some of which was given to other areas in the region).
Some Ukrainian provinces have also been reduced a bit and Kiev has been brought closer in line with other European cities.
The provinces are much larger which probably plays into it somewhat.
Yes. This also has a lot to do with it, both for the provinces themselves and for the countries. A large country will have more total development simply due to its size and the Poltava province for instance is big enough to fit 4 of Brandenburg's provinces in it, which can result in a higher development level in a province - province comparison even if it would be dwarfed by those 4 provinces together.