Builder needs to provide proper documentation on why the area increased and how old and new layouts will change.
Argument that during booking it was signed; "that builder can increase\decerease area" will not work without proper facts from Builder.
Builder need to prove it why the area is increased and where is it going to be reflected"
All builders are aware of this normal practice and hence they craft their BBA accordingly. It will surely have one clause that legally binds you to accept the final size.
Some very profitable way to increase size is to increase balcony size, add few recesses here and there, add few storage space below the box window and so on.
So what builder propose is, the area sold to you is tentative and it will be measured at the final deliverable and accordingly the final size of the apartment will be decided. Technically the area can increase or decrease, both way is possible. However, knowing the way it works in India, there will be only one way.
If you want to know the correct law governing the issue, you may issue a legal notice through a local lawyer to the builder seeking his explanation for seeking additional amount in the name of increase in super area which was not in the sale agreement and deny the payment until he quotes relevant clause in the agreement. He will certainly refer the matter to his legal department or lawyer who will quote the relevant provisions of law, which if not convincing you may decide to proceed legally as per the same law which has been quoted.
These days the developers/builders take the purchasers for a good ride by mentioning lot of technical terms in order to indirectly extort money from them on one or the other pretext, so it is buyer who should question each an every aspect that involves cost.