Area of DaVinci's square pizza = 15 x 15 = 225 square inches
Area of Italiano's 16 inch diameter pizza = pi x (16/2)^2 = 3.142 x 8^2 = 3.142 x 64 = 201 square inches.
Therefore, DaVinci's square pizza is 225 - 201 = 24 square inches more pizza than Italiano's 16 inch diameter pizza.