Throughout the majority of the post-conquest middle ages in England (it was different in other places) the amount of land owned by the nobility is well known. It was zero, not a scrap, not even so much as a grass verge.
The Feudal system as it applied in law as simple. The King owned the land. The king granted land for those he trusted or to those with whom it was convenient to entrust land to. This was in return for services, usually inlcuding, military service or for other types of fee.
When a superior landlord died his heirs would inherit on payment of further fees and with the tacit and express consent of the sovereign - otherwise it reverted to the Crown and would normally be granted elsewhere.
Whilst heritable land and other properties did become the norm towards the end of the middle ages and into the modern era, the basis upon which the system was based was that the king owned it all.