Call Team Estrogen and ask? They've great customer service.
Fitting. If you have access to a Nordstroms, go and request a bra fitting. Also, follow the
guidelines on
HerRoom.
Nordstrom's will bring out a fitting bra, and try many until you find the perfect one.
Bra fit. Okay. With your arms tight to your sides, bring them forward and up. You shouldn't be able to feel the 'girls' with your arms. So, the bra should bring them together, within the width of your ribcage. Also, stand in profile, and look in the mirror with your arms at your sides, and bent at the elbow, forearms parallel to the floor. They should be lifted, so the equator, or fullest part, is about halfway between your elbow and the top of your shoulder.
The band and underwires/seams should sit below the breast-tissue and hug the ribs, by about two inches. They should not touch soft tissue.
Having the 'girls' positioned correctly is more comfortable, and better for your back.
Buy with the hooks on the largest setting, because the band will stretch.
A common problem, bra band number too large, cup-size too small, this results in them sitting too low and too wide. If you can look in the mirror, and see the girls falling too low and outside the confines of your rib/torso structure, you probably need a size or two larger cup, and a smaller band-width.
About 15 years ago, I went to a store, tried on a dress, and was about to put it back because it was too tight. The sales woman sent for the lingerie specialist (from another floor), who came up, took one look at me, and ran for a 34D. I had been wearing something like a 40B or 38C. With the 34D, the dress fit. I bought it.
We all fluctuate over the years. From age, if nothing else. I'm considering looking into 32DD's. Hard to find, though.
I'm not sure about the muscle-size affecting the bra-size, but you should be able to fit this way regardless.
Edit: The discomfort you describe sounds like the straps are providing most of the support. The band should do that, but it can't if it's too large. Definitely sounds like you probably need a much smaller band, and by extension, larger cups
THis page explains how to fit. But every brand and style fits a little differently, so if you order online, be prepared to send them back. Really best to go to a nice department store at least once, so that you can see how it's supposed to fit.