Gynecomastia, or "man boobs," affect between 33 and 41 percent of men between the ages of 25 and 35, 60 percent of 14-year-old boys and 55 to 60 percent of men over the age of 50. CNN notes that although the condition can correct itself, it was the fifth most common cosmetic surgery for men in 2012. Furthermore, research shows that marijuana use could (strong emphasis on could) be linked to the condition.
Irregular levels of testosterone and estrogen cause gynecomastia, prompting the body to create breast tissue when there's more estrogen. According to CNN, studies in animals show that exposure to THC—marijuana's active ingredient—can decrease testosterone levels. However, the effects of THC on human testosterone levels are not as cut and dry.
While a 1972 study drew a link between weed and gynecomastia, a 1977 study of U.S. Army soldiers found no ties between the two. However, the legalization of marijuana in more states could allow for more testing and clear results of its effects on testosterone.
So while smoking a lot of weed might lead to man boobs in some cases, it's far too early to make any declarative statements. Keep on smoking without fear.