Although I do not charge BBW on a sliding scale, I do think that one day this might change. However, for now, I admire your passion Humble Waxer! In this case, I agree with Wax Queen. Charge based on amount of TIME it takes. I have many clients who are on the "heavy" side and some are quick BBW's and others are not.
The way I see it: If any waxer has to take the TIME to trim the hair way back because its a forest, wax an hour of TIME and use nearly a whole can of wax, this is considered a wax that should cost more than your usual rate. This is of course, regardless of the size of the person. I have some "skinnies" that are just as difficult to wax.