You get stretch marks when a part of your body grows faster than the skin is able to stretch elastically. This can happen from a teenage growth spurt, sudden weight gain (ie, pregnancy), or even muscle mass gain. One does not have to be overweight to have stretch marks.
Yeah, there are some male trainers at my gym who have really nasty stretch marks on their biceps, shoulders & chest. It was the first time I've ever seen it, or maybe just the first time I ever noticed it.