The good news is that breast massage feels great and has really no downside risk. Breasts and even more so, nipples are an erogenous zone for most, male and female. To me, that is reason enough to do it, and I do. What's more, if you light some candles, stretch out in a nice warm bath, sip a glass of wine, put on your favorite music, follow with a slow lingering massage... well it feels even better yet.
Whether you will obtain much if any breast development that can be specifically attributable to the massage is up for question. Anecdotally, some report an effect and mechanistically there is some reason to believe this should occur by way of reflex activation of prolactin and oxytocin secretion by breast and nipple stimulation paralleling the effect of breast stimulation during breast feeding. There are accounts of essentially "normal" men who have stimulated development of glandular tissue and frankly lactated in the absence of hormones. The amount of stimulation to achieve this is said to be very extensive something on the order of massaging the breasts nearly hourly all day long. If you are inclined and have the time and circumstance to allow all this massage, it would probably be worth a go at it. I have read accounts of individuals that suggest massage to the point of inducing lactation to help hypertrophy the glandular tissue of the breast as a further measure to induce the development of fully characteristic breast structure of a mature natal woman who has had a prior pregnancy, helping to get past the perky late tanner stage teenage-type breast structure common in MtF development on HRT alone. Some have claimed that this has helped them obtain the breast structure they desire without resort to surgical augmentation. All these were anecdotal reports, I don't know that this has been ever rigorously studied and an shown effective strategy.
Taking testosterone antagonists mainly results in withdrawal of the effects of testosterone but may result in varying degrees of gynecomastia as a result of the "unopposed" action of estrogen, something all men have to some extent albeit at much lower concentration than women. This is a common complaint of patients using mainly spironolactone for other purposes such as treatment of heart failure or after myocardial infarction.
All this said, there is nothing like taking HRT level doses of estrogen +/- progesterone to produce what is essentially female puberty and resulting breast development. Without the estrogen it is unlikely you will have the overall remodeling of the breast with deposition of the typical fat that makes up a substantial portion of the mass of the mature developed breast. There is debate about the effects of estrogen alone vs. the combination of estrogen and progesterone. Most would agree it is primarily the estrogen that is needed to bring about the process leading to fully developed female breasts.
