I'm pre-t, so take this for what's it worth, I'm sure a lot of the other guys can fill in my holes. But T causes fat redistribution, so it will reduce breast size. But breast tissue is also largely ductal and connective tissue, so it won't be as dramatic as most guys would like. If you bind a lot, the tissue can get broken down more in conjunction with T, potentially resulting in what my friend calls from personal experience "pancake boob." Very scientific, I apologize lol. It's my impression from reading and talking with guys that fat redistribution is one of the slower, more gradual changes, so it probably won't be overnight; that being said, it's different for everyone, and it's hard to create a general timeline.
I hope someone posts from experience, or someone who has more qualifications than my glorious mustache. Good luck to ya