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It sounds like you might want to buy your shirts with a looser fit, alright. This is a bit hard to imagine for me, since I've always been the opposite-- everything is too loose on me.
Emily,
Sounds like you're getting some breast growth. Born male mammaries are not as centered under each collar bone as they are with born women simply because our chests are generally wider. Of course that's a generalization, there are plenty of MtF members here with very petite little rib cages. Personally I'm built like a tank and over a period of years my breast tissue started developing more heavily toward the armpits as is evident in the photo, the under breasts creases looked like two J's facing each other. It took several years and the growth of the glandular mass under the nipple to develop sufficiently before the area towards the breast bone began to fill in at all. Here is a photo of me taken last year. I was wearing an underwire unpadded soft bra and a rather clingy Victoria's Secret crew neck long sleeve T- shirt. You'll notice that the nipples do not appear centered under the collar bones like they do on cis women. But prior to all the development as things were beginning to show, I wore tight men's T shirts and loose fitting straight cut men's shirts to avoid outing myself in the workplace.
(https://www.susans.org/proxy.php?request=http%3A%2F%2Fimageshack.us%2Fscaled%2Fmedium%2F199%2Fim001319.JPG&hash=eff35c4027b0ff2198741a401ab477052a0e0b82) (http://imageshack.us/photo/my-images/199/im001319.JPG/)