a 11" is 51 square inches.
a 13" is 72 square inches.
That's 21 square inches difference almost 40% bigger.
Where'd you get those numbers?
I got the 11 inch at 11.8*7.56 = 89.2 square inches.
13: 12.8*8.94 = 114.4 square inches
So the 13 inch is about (114.4-89.2)/114.4 = 22% bigger.