Smaller province and movement areas. It is entirely unrealistic to assume that an army in northrern Aquitane can control southern Acquitaine - or that a control of Cordoba's Atlantic coast equals control of its Mediteranean coast - or that the entire Russian/European borden can be sealed with two "areas", etc. (the smaller areas could be combined with the "highway" idea to allow realistic movement in own territory).

Think about it, it is very easy for England to conquer all of France in ten years' game time. But in real life, it would take ten years to maybe occupy Flanders alone...