When glandular tissue that developed during puberty is maintained beyond a certain period, the tissue becomes fixed. Even when hormonal balance returns to normal, glandular tissue that has already formed rarely shrinks on its own.
If fat tissue increases along with weight gain, the chest protrusion becomes more pronounced, and aromatase activity in fat tissue promotes conversion to estrogen, which can further stimulate the glandular tissue. Hormonal fluctuations may not fully stabilize until the early 20s, so the end of puberty does not mean the end of hormonal influence.