Simply speaking, the round pans are nothing more than shallow cylinders, so I used the formula for calculating the volume of a cylinder to determine the volume of each pan.
Pi (3.14) times the radius (half the diameter) of the pan, squared, then multiplied by the height of the pan. 3.14 x 4.5 (squared), multiplied by 2. The volume of the each pan is 127.17.
Now...the volume of the rectangular pan is the length times the width, times the height, which is 234.
Now, that's just cubic inches, not cups, but it illustrates the volume differences in the pans. The combined volume of the two 9-inch pans would be 254.34, which is a bit more than the 9x13 pan at 234 cubic inches.
Okay, that's me. I'm very mathematically minded and solve many of my cooking/baking challenges this way.
An alternate method is to fill one of the round pans with water to about 1/4 inch from the top and measure how much water it contains, then do the same thing with the 9x13 pan. Simpler, but I somehow always reach for my pencil and paper.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!