Many of us may have been wearing bras with cups that are too wide for our breasts and that's one reason why the wire poke us in the armpit. Those wires could also just be too tall but for the sake of this post, we'll assume that all cups are just too wide. A downside to wearing cups that are too wide AND not swooping and scooping, is that you could end up with migrated breast tissue that never returns home. What do I mean by this? Tissue that should be closer to the front of your chest ends up hanging out under your armpit. A too-narrow cup is the biggest offender for making tissue migrate--think of when you put on some jeans that are too small...where does what doesn't fit go? Outside the jeans in the form of muffin top. You can get muffin top in two main areas in a too small/too narrow bra: your cleavage and your armpits.
Here is one example of a bra that has too wide of a cup for me (the mis-fit is pretty obvious):
You can see where the bra is sitting almost flush to my skin between my armpit and my breast. I think this one makes me look a little shrinkwrapped, don't you? Most of that "shrinkwrap effect" is due to the fact that it doesn't have enough projection at the bottom of the cup but it's the only photo I have of an obviously too wide cup.
So? How can you tell the width of your breasts?
From a standing position, you might be able to tell by where your ribcage starts to present behind your breast tissue. I'm not able to tell from this position but I know others have been able to do so. Here's a sketch to illustrate what I mean:
The top sketch would be considered narrow. The bottom one could fall into the wide root spectrum. I determined my own width be leaning over, with nothing on, and checking myself out. My breast tissue starts in front of my chest wall. None of it attaches under my armpit. Therefore, I search for medium to narrow cups.
I have to make a slight reference to other aspects of fit so that the experts don't start nay-saying me. In the sketch, above. I say "could" fall into the wide root spectrum because they could also just be *wide-set* not wide rooted. I'll save that for another post though.
So, back to my lean over assessment. Here's a pic of me doing this assessment. Please note that, yes, the boobs are covered. However, this tube top that I've slid over my breasts like a sock does NOT pull them in. They are still dangling free.
I hope this helps you with shape determination.
As a sidenote: If you have narrow to medium roots, Elomi is not a brand for you. :)