Edit: Darn... Squirrel beat me to it!
Well, I think you really need to count for surface area (square inches of pan) and thickness
If thickness were equal... then you would just need to deal with suface area... = pi x radius squared
12" pan = 6" radius Surface area = pi x radius x radius = 113.09 square inches
10" pan = 5" radius Surface area = pi x radius x radius = 78.539 square inches
If 7 ounces spread over 113.09 square inches (12-inch pan), then
ounces over 78.539 square inches (10-inch pan)
Yes, you would use the same fractions like you did
7/113.09 = x/78.539
7x78.539 divided by 113.09 = 4.861 ounces...