Some mutant have glandular problems that can effect growth, as well as some races of mutants that have become smaller or bigger over generations. The following mutations effect size:
Shorter: This mutation causes dwarfism, or makes a mutant proportionately smaller. This usually make the mutant sorter then a meter (3 feet). This mutation decreases the metabolic rate of the mutant and also causes a marked reduction in its ability to do damage in physical combat, as well as being harder to target by larger opponents.
Taller: This mutation causes gigantism, or makes a mutant proportionately bigger. This can yield a being ranging from 3 to 8 meters tall (9 to 26 feet). This mutation increases the mutant's damage potential, but becomes an easier target.