There is nothing wrong with the computation. Observe that you are talking about area here, not length.
Lets perform a small approximation
What would be the area of a square of side 16? It will be 256 square units. Your circle is entirely inside that square (that is why I chose 16). So it should have lesser area. And ya it does!
The moral is to compare the suspicious numbers by approximating it with a reliable way of computation.