If exercise makes the area thicker instead, muscle is likely the cause.
Medication in the target muscle reduces its force, and a muscle that goes unused gradually gets smaller so the line slims. It only makes sense when muscle rather than fat is the cause, so consultation first checks which is thicker. If the two sides differ, the amount is set differently.
One session lasts three to four months. While the drug blocks the nerve signal the muscle goes unused, and an unused muscle gradually loses volume. When the drug wears off that lost volume does not come straight back, so with repeat sessions the slimmer state holds for longer each time.