You're not eating enough. I find it really easy to manipulate weight both ways.
"You're not eating crappy enough stuff. After stuffing myself full of sugar and fat i find my genetics to end up changing my weight to some direction even without adding muscle mass." <-- fixed that one for you ;)
me myself, atleast if i dont eat pure sugar & fat diet, wont end up gaining weight with basicly anything but having my muscles grow, in the genetics.
good thing i dont really need to get more mass, as i really dont like eating much sugar, i dislike its taste (and no sugar does not taste sweet, it tastes like sugar and dampens all other tastes in food, unlike making foodstuff sweeter with several types of fruits and so...)
basicly no matter how much i eat, it wont stick as fat. (im thin next to having no stomach to be found if i havent just stuffed myself full of food, but still i dont basicly know anyone who can and does eat as much as i do irl.)
but muscle on other hand is completely different thing: back at military service first 2 months most reported loosing weight, i gained 3kg (fastest change in weight in that decade) and during 1 year of service gained total of about 10kg (when i entered i was about 54kg [and just under 180cm if someone is wondering]). later after service at some point i picked up fencing again gained 9 kg in mostly just leg muscles in year or two.
Poll