Streched skin will not ever shrink back into position. It's like the elastic that has been worm out in an old pair of under wear, when its gone its gone.
Now, you can fill the bagging parts with bigger muscles and reduce the look of the sagging skin ,but it will never return to it's old look... Have you ever seen an old body builder in their 70's? They have the same bagging/sagging issues. They stretch their skin with their muscle then when they get older and stop working out the muscle start to shrink and the skin saggs..
When I was in high school I played high level Football and worked out feircely 5 days a week! My biceps were 20" around and I could bench press over 400lbs for 5 reps and a set of 3.... I squated over 500lbs and my thighs were 28" around and I only weighed 200lbs... I had stretch marks on my biceps and my thighs from the muscle streching the skin. I still have the strech marks today and much worse from all the fat....
I went from 200 lbs of pure muscle with 5% body fat to 470lbs and 70-80% body fat in 25 years.....