Lean tissue weighs more per unit volume than body fat.
Laboratories use this difference to determine people's body compositions, using a technique that involves weighing a person submerged in water and comparing that to their weight out of water. This process, called hydrostatic weighing, estimates body fat percentages by computing the overall density of the body. Conversely, you'll weigh more underwater if you have a higher percentage of lean tissue, which sinks because it's more dense than water is. If your BMI is over 25, you are considered overweight, and if it's over 30, you're classified as obese. But his chubby counterpart could easily weigh less, which might make his BMI less than 25.