I also have seen people go to Ann Taylor and other type stores. Homegirl, that got the Bart Simpson, went to H & M. I think that they point them in the direction of stores, yet they may have a little free will. I think that they have to go to certain stores for the video footage and they may have to do a few "plugs" for these higher-end stores (not that they really need them

).
I think it's about quality and fit. One thing that I know it bigger people CANNOT wear cheap clothes. Cheap clothes are so unflattering. Therefore, they benefit from quality clothing. Also, sometimes higher end clothing lines fit like gloves on some people. They are usually tageted for smaller people and they usually fit them well.
Aside from that, if I had a free $5,000 I might splurge on a Prada shirt, bag, or pair of pants just because! Hey, when else will you get the opportunity?