I have a few more questions! What are the methods to increase supply limit on enemy territory? are they proportional to your land's supply limit and goes up when your land's supply limit increases?
Short answer: The only way you can affect enemy province supply limit is by sieging and controlling it.
Long answer: The supply limit of a province is based on it's base tax, fort level, the owner's land tech level modifiers and other modifiers.
It is also multiplied according to its status.
-If it's your own province, it's multiplied by 5
-If it's an enemy province, it's multiplied by 1
-If it's being besieged by you, it's multiplied by 2
-If it's controlled by you, it's multiplied by 3