I don't think admirals are having a special, but mysterious type of genes that make them taller and sane, I researched about gigantism, it's considered a disorder in real life, they may sometimes short-lived or even drive them insane, but when it comes to One Piece's overly 250 cm characters, I think why they live normal than the rest of them, having higher longevity (Edward Newgate is surprisingly 75 yrs old) than real life counterpart.
yeah real life humans aren't made to be as tall as some one piece characters, i don't think our bones are strong enough to be able to handle that much weight, in addition to the sanity thing and shorter life span. our bones would just break and you would never become the world's strongest man like whitebeard.