HRT will (probably) make you lose muscle mass, as well as gain fat. Since muscle is heavier than fat, you SHOULD lose weight. Ho-we-ver, HRT reduces your metabolism to female levels. It SHOULD reduce your appetite to female levels, too, but since appetite is vastly psychological, you have to be careful, or you'll put on weight.
Your shoulders (unless you're below 16, and even then) shouldn't get thinner, in terms of bone. However, the loss of muscle can reduce them more than one would think, depending on the amount of muscle there is. And even if they stay a little broad, if they get less muscle on them, they might seem very feminine anyway,