As the question says the size cans are similar, we'll build up a proportion:
Large one: 5 inches, holds 12 ounces
Small one: 4 inches, holds x ounces.
So:
5 --- 12
4 ----x
Doing the cross product:
5x = 48
x=48/5
x =9,6 ounces
The small can could hold 9.6 ounces of soup.