Just like any other physical phenomena, there are going to be slight, moderate, and extreme cases.
In a young man, gynecomastia (gyno) can happen simply as a result of his testicles turning out large amounts of testosterone (T) too quickly. Other side effects in young men related to increased T production are acne vulgaris, hair on the face, accumulation of muscle mass, and deepening of the voice.
In super-huge men (aka bodybuilders that are abusing anabolic steroids) it is possible to see a ridiculously muscular person with puffy bitch nipples. It's a nasty way to ruin an otherwise good physique. Steroids are bad, please do not use them, just commit to lifting for many years and enjoy your incremental gains along the way...
The [NASTY] picture that has been [EXCESSIVELY] ququoted in this thread is most likely a combination of genetic predisposition, too much fat in the diet, and a complete lack of exercise. Poor fellow needs to right his ship!
When I was 16 I had a slight case of gyno in my left pectoralis muscle, it was a slight accumulation of fluid under my nipple. I went to my doctor who asked me if I was using steroids and I told him no and he believed me as my weight had not changed significantly since my last visit and I exhibited no other side effects as such. I HAD TO GET A MAMMOGRAM! <so fucking embarrassing> After that came back negative for breast cancer my Doc asked me what I wanted to do and I told him "Get rid of it!" So he numbed the area up, extracted the fluid, and it never came back again.
THE END.