Will the new collagen developed by the Sculptra shrink proportionately as I lose body fat?

Sculptra makes collagen and collagen doesn't shrink with your fat. If anything having Sculptra will make it look better.

You do not post photos, but your story is enough for me to tell you the truth: There is no way that any filler can fill that much volume back into your buttock or hip dip area. Remember, we use anywhere from two to ten vials to volumize the face, a structure that needs to have definition of it's substructures to look normal. Most people in your position are in need of volumes north of a hundred cc's or even much more. How many vials of Sculptra is that? Well, no one knows for sure, but it is far, far more than the usual 20 vials most people can afford to purchase. Please do not waste another penny on Sculptra. Go ahead and lose the weight and when you're done, go to a board certified plastic surgeon to be evaluated for your issue (and not the one who sold you the Sculptra, if that's the case). At the very least, you'll need a combination of peripheral liposuction and fat grafting of the defect, and you'll possibly need something more (skin excision). Once again, I can't see you so I am just giving you statistically likely scenarios.