The axillary tail of the breast expands into the armpit. Depending on the composition, fat versus glandular tissue, treatment may vary. In my experience excision is more often necessary.
Lateral chest rolls are quite common after reductions and if you were overweight, even more so. If you lost a lot of weight, then you must have some component of excess skin and in this scenario, excision with possible liposuction will be needed. If you are just full and skin quality good, and my patient, I would suggest liposuction first and then see if the skin needed to still be removed. So the answer is 'it depends' on what we are starting with. You should be able to discuss all of the pros and cons with your surgeon and reach an agreeable plan.