Overworking the dough, which develops gluten, a sticky substance in flour, causes shrinkage and tough pastry.    After rubbing in the butter add water or egg yolk gradually; keeping the dough loose until it can be drawn into a ball with little pressure.  Knead gently and roll the dough. 

 

Next the best route is to always shape the pastry before chilling to allow the pastry to rest.  This helps reduce any shrinkage that may occur during cooking.