alyssa,
Edited my original post after realizing I violated TOS rules regarding posting links, by including links to the sites I now describe below. Sorry about that.
I am a manual physical therapist and know quite a lot regarding the effects of massage/body work on the body. I have also researched fairly extensively natural breast enhancement. There are websites, analogous to this one, where this is discussed (I'm not referring to advertising sites for specific products.) Many cis females claim to have achieved some success with these methods. Those websites have sections for biological males wishing to enlarge their breasts. Many cis males also claim success, with pictures to back up their claims. There are MTF's who are using these methods either in conjunction with HRT or in lieu of it for various reasons, who have also posted photos. Obviously, you can't believe everything you read on the internet, but before you pass judgement, you should do some research. Apparently HRT and surgery are not the only ways to enhance the breasts.
Massaging the breasts is done to increase blood flow, with the idea being that increased blood flow will help maximize any potential for growth. In everything I have read, massage alone will not increase breast size, especially in biological males. Massage is recommended as an adjunct to other various methods.
If your chest muscles (pectorals) are sore, you are massaging too vigorously and should scale it back. I seriously doubt you can self-inflict enough tissue damage to cause a visible swelling in the breast tissue with massage. This would almost certainly involve way higher levels of pain than just soreness, which of course, tends to be self-limiting.
