Every time X goes up 1 in the first example, the amount that's actually changed is different. In the second, it's the same. I've eaten 1/5 of the pizza. Now 1/

Every time X goes up 1 in the first example, the amount that's actually changed is different. In the second, it's the same. I've eaten 1/5 of the pizza. Now 1/4. Now 1/3. Slices are all different sizes. I've eaten 20% of the pizza. Now 40%. Now 60%. Slices are the same.