I have fairly muscular arms and shoulders, as a result of my weights routine and inversion yoga. This is most clear when I flex them, or if doing anything with my arms up like tying up my hair. But if I'm just standing or sitting with my arms relaxed against my body, I think they look big and flabby. As soon as I move them or put my hands on my hips you can see the muscle again.
I've gained a little bit of weight this year as a result of months long illness and I'm wondering if that 'flabbiness' is a result of a little excess body fat or just what relaxed muscle looks like and I never noticed before. (Because I was doing a lot less workouts in my bedroom in front of a mirror.) I lost muscle too while ill but I've built that back up. I'm not overweight now by any means, BMI of 22.5 with plenty of muscle, but I am carrying about a half stone more than I like best for me. And it's very stubborn about shifting.