I had a horse who skinned his stifle (just part of the skin, not all the way through) an area larger than my spread hand. As it dried and formed a scab it shrank down to the size of a toonie (Canadian $2 coin less than 2" diameter). It took several days to soften and get off so he could move his leg properly. After that I kept it coated with zinc ointment to prevent scabs forming.
My guess is the edge of the upper skin flap is dying, drying up and pulling tight. It looks like there might be some crusty, scabbing along the edge in the photo. If you can get the scabs off and keep them from reforming, that flap will probably stretch back out and cover more of the wound.
My skinned stifle horse had significant swelling around his stifle (three grapefruits) and it took months to go down (happened in November, and the swelling is still visible in February photos). I used ice, bute, and exercise - exercise worked best.
Good luck with your filly! The progress looks great!