I'll slip in here, because this is an issue I have worried about. Sorry about the length.
Don't get too worried, and certainly not too soon. To me, there are two aspects: what your regimen actually does to your body and thinking in terms of proportions, not size. Of course, one must approach it based on your own case -- for how you both diet and exercise. Also, the end result is not about chest size or weight, really. It's how you are shaped and proportioned -- and that also is affected a bit by diet, HRT, and exercise. My approach was to break down and build back up -- but build up with the HRT going in full and a targeted exercise program for muscle. For the chest, it meant losing bulk and trying to shape for final proportion there and in other places.
In my experience (which is now far into transition), diet and hrt did more than I expected to reduce "chest bulk," partly because I didn't realise what size I really was underneath. I assumed I had a large frame. In fact, when I did the wrist and elbow measurements, it seemed I had a small/medium for a male (under 6" at the wrist when I started), although I was tall (5' 10") and fairly broad shouldered. I went from a 44+-ish chest (as in what would be measured at bust) to 37. I think my ribcage size went from 40" to just under 35". I had I assumed I was a"big" person underneath it all, but it was just that I carried it all over. I managed to go from about 205 to 149. To me, HRT had no effect on my ability to lose weight (and I am in my early 50s). More important was the type of diet and my willingness to do significant cardio every day.
For diet, in consultation with my doctor, I started by figuring out what a reasonable and healthy target weight for my height would be -- at least when I started. As I went on this changed -- down as I discovered that I was actually smaller than I thought underneath. I came up with the right diet for me (which cut out carbs, and adjusted proteins and when I ate what). As for exercise, I focused on cardio for a while to both burn fat, and to eat up protein from muscle as the estro/spiro had its effect -- all over (though never cut out the protein intake). I kept adding toning and light strength training.
The other thing we overlook is proportions. There is no question that the primary "male markers" are chest/shoulder and height. These are not end all. Size is less important than is proportion to the rest of your body. You cannot change frame, but you can to some extent affect what's on it and help change proportions to those within or closer to natal women. You can try to focus on waist and hips workouts, as well as toning upper muscles as HRT has its effects to soften it all. While you can't widen your hip bones, you can build larger muscles that attach to them below, and that creates new curves for you.
We often hear that "natal women come in different shapes," which is true, but they are no happier about it than we are. If a natal woman has an inverted triangle shape, it's often softened by her waist and hips. They do not look less feminine in their triangle, it's just not as pronounced and offset a bit by other, female, markers. We can often influence it the same way. In addition, many female athletes have this issue, and they do not look less female either -- their look is the sum of their estrogen-affected muscle shape, and other "softeners," etc.
Finally, I happen to have an arts training as part of my background. There are plenty of diagrams out there about "ideal proportions" (lots online). NONE of them address size. Instead, it's how many head lengths or widths etc., or hand breadths, in proportion to the rest of the body. While we hear "36, 24, 36" or whatever as an ideal set of measurements, that's meaningless in many ways. They don't look the same on everyone or at all heights. As it happens, I'm 37, 29, 37, which might be thought of as slightly hourglass, but I don't look that way at all -- my shoulders are broadish, and my waist isn't evenly "small" all around my hips. I have a softened, mildly inverted triangle that I work at, and that helps.
We all worry abut this. In my view, we can take some steps to improve our look, but at a certain point we cross the line from wanting looking like who we are (female without a thought) to wanting to look more a beautiful ideal. The latter is fine, and we all want that, but it starts to get very subjective very fast. It's also very individualistic based on where your body type and shape comes out. To me: focus on getting proportions and shape sorted, don't worry about numbers.
