Measurement of water tank

Construction of my house is complete, contractor is claiming covered area of water tank double than the actual tank size. Nothing is written in the contract like this but contractor is saying it is a normal industry practice. Can some construction expert advise me on this matter.
Thanks
Arif

Water tank is usually measured in gallons and not square foot. That might be the difference

What will be rate per gallon or per cubic foot, if walls are made of bricks and top and bottom are made of concrete slabs.

70 -80 per gallon