I would not sweat it too much. When I was in my mid-twenties, I lost +/- 50 pounds, and at first my skin was a little loose, but I worked out, especially strength training, and after three to six months or so my skin was perfectly tight again...although my breasts were never quite the same.
Victoria's Secret can fix that little problem...
Now that I've had my kids, I have just lost 45 pounds again in my mid-thirties. So this time my skin is not rebounding quite as fast...but it is rebounding. It is just taking more time, and I am pretty sure that my tummy is never going to be the perfect thing of beauty it once was...but it is close and its getting closer. Of course, we won't talk about the state of my chest after nursing three kids and the 45 pound loss...but, hey, I gave life...and Victoria's Secret is still my friend. Everywhere else though, I really think the strength training is helping...good nutrition is also a must.