The armpit fat area that sticks out of a "bra" is unofficially called "bra bulge". I just seen on Dr. Oz that is can be permanently removed for $1500-1700, the reults were really good too. So if you've tried losing weight/body fat and that fat never goes away, here's a simple, quick, non invasive solution.

I've seen guys with binders who are skinny(luckys!) and they have no arm pit fat popping out, but guys like me who aren't skinny have to deal with it. And it bugs. So just thought id share this info with you guys.

Lol....bra bulge...

I'm skinny and I get it.  its gross

Ew, I've got bra bulge.  I think the only solution is to work out like mad, I doubt dieting would work unless you starved yourself.