Gentlemen,

What they are talking about is referred to as gynecomastia (male breast growth). Gynecomastia results when male androgens (such as testosterone, DHT, ect) are lowered allowing natural occurring estrogens and progesterone to have more influence in our bodies (androgens are antagonists to estrogens so when androgens are lowered our 'female' hormones start exerting more of an 'effect' on our tissues).

This actually is not BS but it doesnt happen to everyone and isnt limited to marijuana use (as gornik said alcohol can also have this 'effect'). Typically such problems occur in certain predisposed individuals who typically are heavy users.