Why would you expect Scotland, which has few units and poor provinces, to do well? In about 1 in 20 cases it will get 5 or 6 provinces which is apparently due to England being unlucky and fighting on several fronts or being excommunicated. This seems to be just about right to me. In my present French campaign the Scots held 5 provinces and then stupidly came over to gratuitously attack me in Bruges. If they somehow manage to get York and Nottingham they can field some decent large armies.