Punjab has always been properous in comparison to the other Provinces, oweing to the fact that the British invested a considerable amount in Agriculture etc. Hence Punjabs economy has always been relatively robust.
It was only natural that Punjab would take the bulk of the resources as its economy was always stronger then the other provinces. Larger economies demand more resources.
Thats not to say that Punjab doesnt dominate unfairly. But this is due to circumstance, not by design (Atleast it wasnt in the begining..)