Most medicals are done by doctors who have no training in diving or hyperbaric medicine and are unable fully to assess the risks. The idea of the BMI is to determine which divers need to be referred on to a doctor with these skills. When I see divers with high BMIs, I look for general fitness, flexibility and experience. In most situations it is not.
People lie on the forms, and also during the medical.
I believe the medical should be scrapped as a routine measure and replaced by a PADI-style questionnaire. Only divers with problems picked up on the questionnaire would need to pay to see a doctor versed in diving medicine. My husband runs a licensed charter vessel and takes parties of up to 10 sea anglers and divers out around Oban and the Sound of Mull, on the west coast of Scotland. Some angling clubs make it clear that they will not book with us unless we can satisfy them that we are correctly licensed and that they will be covered by adequate insurance. They also seem to be on the ball about where to check such details.