Why do i have man boobs even though i'm not fat?

Many causes: There are many causes for this. Some of these are serious conditions that need to be evaluated and treated accordingly. These causes include: malnutrition, alcoholism, liver problems, drugs (legal and illegal) side effects (spironolactone, aids meds, marijuana, amphetamines, ), prolactin overproduction, low testosterone, kidney failure, thyroid problems....

Drugs/alcohol/none?: Multiple reasons. Usually some medications, drugs, alcohol are behind this issue but it may present without any particular cause.

Not uncommon: Many patients with gynecomastia are not overweight. It's caused by a hormonal imbalance usually. Many young men have a mild case of gynecomastia, with only puffy nipples. Others have a more serious case. Most require surgery for correction, even mild puffy nipples. See my webinar on youtube - search dr. George pope. Or visit www.Gynecomastia.Org, a great forum.