I haven’t done the AT. Both of my 2 PCT thru-hikes i lost 35lbs. First time starting at 188 ending at 153. Second time starting 193 ending 158. The difference for me was a better plan for after the 2nd hike. First hike afterwards i gained back everything plus 10lbs. Second hike i gained back most but not all. Actually since that 2nd hike i still haven’t gained all the weight back, by being smarter about what i’m doing to keep in shape.
My guess is that it’s at least as easy to lose weight on the PCT because you have a couple of legit sections where you can’t make up for calories in town. I.e. long stretches without town food.