If you start wearing the right bra size you can generally get rid of the armpit fat, because it's probably lost breast tissue!
I had that issue recently because I had been wearing a 34D bra when I should have been wearing a 32DD (I shrunk out of the 34D and was too lazy to go and get a 32DD...). At first when I put the 32DD I would get spillover on the band, but I would make sure to bend over and scoop all of that into the cup. Now it's just about gone and I've noticed that my measurement around my rib cage went down slightly (a little over 32" to just under 32") and my breasts seem much better and fuller.
Throughout my weight loss I've found the best way to measure my bra size is to measure without a bra on and measure underneath my breasts, which gives me my band size. Then I measure over my breasts to get my cup size. You might get better results with a bra on, but if it's a poor fit you might get the wrong measurement!
I measure ~32" for my band and 37" over my breasts. That makes a 5" difference so I get a 32DD bra