This answer to this question is why I invented the VARIABALL ®!
The short answer is you need a range of medicine ball weights. Just like you need a range of barbell or dumbbell weights appropriate for the exercise and number of repetitions you plan to do and where your current fitness level is. If you're doing a high number of reps, the lower the medicine ball weight you'll need to use. Conversely, if the number of reps is low or you're looking for a challenge, you may want to use a heavier medicine ball. A person with a body weight of 140 lb (70kg) or less typically does using a medicine ball in the weight range of 8->16 lb. People with higher body weights (and/or advanced fitness levels) can consider using medicine balls of 14 lb or greater. The VARIABALL product line has an overlap in weights at 14 and 16 lb to help make it easier to decide which VARIABALL model is best suited for you: the "Blue"- 8, 10, 12, 14, 16 lb or "Black"- 14, 16, 18, 20, 22 lb.
Before the VARIABALL was invented, I quickly found that I needed a different weight medicine ball depending on the type of exercise I was doing and the number of repetitions. For example, during a "wall ball" WOD at Crossfit, especially when I was starting out, I needed a lighter weight like 10 lb!! As I progressed toward a Rx weight of 20 lb, I used medicine balls of different weights (12, 14, 16, 18, and finally 20 lb) over several months to build up my strength and stamina. Other medicine ball exercises (Russian Twists, weighted situps, etc.) required different weights. When I looked into buying Crossfit quality medicine balls to practice with at home in this weight range, I was astonished to find it would cost me over $500!! The trick was to design a SINGLE variable weight med ball that would stay balanced (i.e. not "wobble") as the weight was varied over a reasonable range of minimum to maximum weights. AND it needed to be durable, AND easy to manufacture, AND priced reasonably, AND feel great when catching it. A couple of years later, I was done :)