Physical therapists often use ankle swelling as a way to gauge treatment results after a sprain. If the swelling goes down, a therapist may assume the ankle is improving.
A recent study showed that swelling isn't necessarily a good measure of improvement, however. Among 29 patients with ankle sprains, swelling (as measured by a "figure-of-eight" around the ankle) wasn't related to the amount of pain patients had or their ability to do daily activities.
Swelling is just one measure of recovery. Talk with your therapist about the problems you have with your ankle from day to day. When it comes to gauging your recovery, your experiences are more important than what a measuring tape can tell you.