Unfortunately, HRT is not going to do anything to reduce your bone structure. But, back and shoulder muscle, as well as upper back fat, will reduce in mass. That will provide you the appearance of slimmer shoulders/torso. Regardless of how much you bulked up or didn't as a man, the muscle mass and fat is influenced by Testosterone and you will have extra bulk on Testosterone than Estrogen. It takes a while for the muscle to break down and become less prominent than the fat, but it'll happen.
Fat migrating to your lower body will also help to minimize the look of your shoulders, by increasing you hip proportions. That is also a pretty slow process and you're likly going to be on HRT for a few years before you see much in that area. Sooner is always possible, but it all depends on genetics and when in your fat "lifecycle" you started. Changes could come sooner or later. If you're in a hurry, you can always look into the brazilian butt lift (my personal favorite), or hip/booty implants.
Being younger, your growth hormone levels will be a bit higher and less time on Testosterone will mean quicker action and less Masculinization to reverse. So, you should see some notable changes in fat distribution around the 2 year mark. Best guess from what I've read from younger posters.
