You really will not attract fat girls for clients by showing fat girls in your portfolio b/c fat girls are not looking at your work thinking, “wow she really made her look thinner than she was, maybe she can do that for me.”
It’s not politically correct to say that, but it does make a lot of sense from a logical perspective. If you see a photo of anyone you don’t know, what do you have to judge them on, except their image in the photo? If you are shown a portfolio of heavy girls, how are you going to judge if they actually look better or worse than if they were in the room, if you have never seen them before? On the other hand, if you are given a portfolio containing photos of several girls and they all look like they belong on the pages of Seventeen, or whatever magazine is current now, you may not assume they were they were all super skinny to start with, but you will probably be impressed with their appearance and by the photographer because of their appearance.