It would be the difference in their circumferences.
So, Circumference of Large hamster wheel = 2 * 3.14 * 6 = 37.68 in
Circumference of small hamster wheel = 2 * 3.14 * 4.5 = 28.26 in.
Difference = 37.68 - 28.26 = 9.42
In short, Your Final Answer would be: 9.4 inches
Hope this helps!