First look: More than 324 square feet :)
Now, [tex]l=2\pi*r[/tex] which means that [tex]r=\frac{l}{2\pi}[/tex]. But l is the original perimeter, 4 times the side of the square, that is, 72 ft. So, [tex]r=\frac{72}{2\pi}=\frac{36}{\pi}[/tex]
Now, the area of that circle is [tex]A=\pi*r^2=\pi*\frac{1296}{\pi^2}=\frac{1296}{\pi}[/tex], which is about 412.5 square feet. It's true, that's more (that's the most you can actually get with that amount of fence :) )