I think you might be able to speed up loss of muscle mass by doing cardio exercise that develops endurance such as running...
And avoiding exercise that strains the upper body and/or involve sudden burst types of exercises, such as sprinting.
If you see runners who run long distances develop a skinny weak upper body whereas sprinters have huge chests much like body builders.
You might also want to consider building your lower body strength to take away from the upper and also help to shape a thinner waist to go from inverse triangle shape to a slight hourglass shape. (won't be amazing because of skeletal limits)
This seems to me like a good routine for building up endurance when running.
http://running.about.com/od/trainingschedules/a/3weeksto30minuterunninghabit.htmAnd this worked really well to trim my waist and make my body look more like an hourglass.
It's also about what you eat, I guess. Maybe the body will tend to develop more muscle mass if you eat lots of protein. Consider trying to focus more on fresh fruits as a source of energy and lowering your protein intake if it's pretty high.
There also other options detailed here. Try searching "body feminization exercise" to see the full discussion. I'm only posting here what seems to be working for me. Again, the results are not amazing because of skeletal limits, but I think it helped get the most of what I've got.
And don't take my word for it, read what others write too. I'm still a transition noob, also 3 months on HRT. For me at least, the above exercise plan of running and "feminizing" exercises focusing on the lower body and proper eating help a lot to shape my body very quickly to the way that I personally want and my dysphoria is more focused on my face. As always, YMMV and what you want to do, what you like to eat - may vary from me - and it's very important to find something to do that's fun! I wouldn't do these exercises if they didn't make me feel good.