Well the author here seems like they want to present this story as a part of human evolution and the two MCs are supposed to be mutated half-breeds. Even fully developed omegas are supposed to have larger frames than ordinary humans but the MC has mutated genes so he is lanky with sickly looking under-developed body and bodily fuctions. While ML has a fully developed physique, he lacks in other bodily fuctions.
Jesus Christ the proportions in this bl is ridiculous on so many levels, not that petite men or wide men don't exist but damn