They're naturally small built with long, lightly muscular limbs. The have a fast metabolism so the can stuff their faces without putting on weight.
To gain muscle they need to eat alot and do heavy weight training, its advised that they avoid cardiovascular activities since cardiovascular activities burn fat
Keith: Yeah I would help him out, but I don't care. He can do whatever the F**K he wanna do!
Insecure Male: Its because of my genetics. I'm an ectomorph!