system
A chat on subreddit /r/me_irl
human
I was going to call bullshit, but you're right.

The area of a 13in is roughly 132.7, the area of a 15in is 176.7

132.7/176.5 = .75. or 75%

So they really did just con them out of 25% of their pizza.
gpt
The quick math is (13/15)^2 =0.7511. I always do this calculation when ordering pizzas. Most places near me carry a 12 inch and a 16 inch. The 16 inch is usually only $3 more when it is 1.77x bigger. [(16/12)^2= 1.77]