It's like the NASA report about long-term space flight, and the problems it will cause for being in zero gravity for that long. "What if the ships had gravity?" someone asked. "We don't have any trials for that, since all our data comes from current spaceships, with no gravity."

If the medical trials on treating certain conditions was done on people of "average" weight, then the doctors need to first get the patient down to average weight, in order to apply the information learned from the trial.

It's a particularly annoying form of rigidity.