Some mutations can effect the metabolism of a mutant. The following mutations effects weight:

Fat Cell Accumulation: A mutant with this problem will be obese (at least twice as large as normal). This mutation impairs the mutant’s movement and fighting ability. It is possible that only certain body parts, rather than the entire body, might be affected (such as the head, one arm, one thigh). This mutation could cause the mutant to suffer from hot or cold weather.

Weight Decrease: A mutant with this problem will be noticeably underweight or outright emaciated. The mutant can eat normally, but not gain weight. This weakens the mutant so that it is slowed in every endeavor, with its physical strength reduced, as well. Without any natural insulation (like fur), this mutant will suffer from the cold.