I'm interested where that conclusion came from?
Please take a look at my thoughts and correct me if I am wrong.
Estrogens derive from circulating androgens in men. Aromatization of the androgens, testosterone and androstenedione, to form estradiol and estrone, respectively, is the key step in estrogen biosynthesis. Obviously, exogenous Testosterone usage can result in the same effect. Elevated estrogen levels are usually the primary factor associated with Gynecomastia. Prolactin is another factor that should not be overlooked. Neither estrogen alone nor estrogen plus progesterone can sustain breast development without other mediators, such as GH and IGF-1. Therefore, it would make sense that using IGF-1 could potentially result or increase the developement of gynecomastia in the presence of elevated levels of Estrogen and/or Prolactin. The only time where I could see IGF-1 directly causing breast developement would be in women. In women, Estrogen promotes GH secretion and increased GH levels, stimulating the production of IGF-1, which synergizes with estrogen to induce ductal development.