smc

The area of the larger pizza is $16\pi$, while the area of the smaller pizza is $9\pi$. Therefore, the larger pizza is $\frac{7\pi }{9\pi }\cdot 100\%$ bigger than the smaller pizza. $\frac{7\pi }{9\pi }\cdot 100\%=\mathrm{77.777....}$, which is closest to $\boxed{78}$.