Area of a Circle

Rory wants to get a cheesecake for her best friend Mariann's birthday. Rory goes to the bakery and picks out a strawberry cheesecake. However, she can't quite decide which size to get.

The smaller one has a diameter of $6$ inches and the bigger one has a diameter of $12$ inches. The baker says the bigger cake is twice as expensive as the smaller cake since it is twice as big.