The theory is that a heaver person, who has much more fat, can more easily lose 10lbs than a lighter person who 10lbs is 8% of their body weight.
A 300lb person minus 10lbs lost 3%
A 150lb person minus 10lbs lost 6%
It is harder for the 150lb person to lose 10lbs than the 300lb person. The body type this does not favor is the muscular one. At 190lbs / 15% bodyfat I could lose at most 28lbs to be at 0% fat. A normal 190 person at my height and weight is 30+% which is 62lbs of fat to lose. (Muscle is more dense than fat) So it goes.