1.5 years ago I had 20" shoulders (it meant L clothes - and not because being overweight, but anything else looked too tight). When I kept looking in the mirror, shoulders and upper arms were one of my biggest concerns (after face) - I am ~ 16.5" now, S size for andro/unisex, somewhere around M/L for female clothes. Of course, when I am standing next to petite, fragile cis-woman, I see how bigger I am and there is limit how much smaller one can get, but hey, when I am staring at my male friends, they seem so large in comparison (and they used to be same or even smaller build-wise...). So, dont worry that much about shoulders and muscle mass, in due time it will melt away and if it works for You - You can always copy the fashion style from those tall, athletic amazon-type females.