Plastic

Arm lifts are called brachioplasties (doctors have a different word for everything!)
 
Think of this surgery as a tailor thinks about taking in a dress, or suit.  The material is the skin, therefore the more extra material (skin) there is, the longer the scar (or seam) is.  The Plastic Surgeon's job is to assess how much the skin will shrink with liposuction versus having to surgically remove skin.  Based on that assessment the surgeon will offer the appropriate procedure.
 
In a patient with excellent skin tone liposuction can be performed alone.  As skin tone diminishes skin needs to be removed, resulting in scarring.  If only a minimal skin resection is planned the scar can be hid in the arm pit.  If more skin needs to be removed the arm pit scar can be extended slightly down the inside part of the upper arm.  When a full brachioplasty is offered the scar travels down the inside of the upper arm to the elbow.  Although this is a long scar it's only offered to patients who otherwise wouldn't get the cosmetic result desired with any of the smaller procedures.  This group very often includes bariatric (massive weight loss) patients.
 
With all of these, tailoring, surgeries every effort is made to make the scar as inconspicuous as possible.  This is done by using fine technique and by hiding them on the inside of the arm.