I think the families involved probably know what they are letting themselves in for - so I dont think its "exploitative" in the sense that they are leaving themselves open to damage, but I do think that its exploitative in the name of entertainment. It wouldnt attract any attention (for "attention", read "ratings") at all if it was just some nice people kindly advising some overweight people delicately that it might be healthier for them to not be quite so heavy as they are, it just wouldnt make good telly.
Because this chap is blunt, it amuses the sort of people who wouldnt be brave enough to make that sort of comment themselves even if they might think it without fifteen pints down them, and the people who think its ok to shout abuse at random people they pass in the street as long as they are in a car that can get away from the "victim" fast, as it gets them brownie points with their mates...
Its much akin to the audition stage of the Xfactor, which wouldnt have been half as successful if it had just been about watching good/better than average people auditioning for a singing competition, they had to have the contrast of the "failed" auditionees to increase the interest because people like to see other people fail, it makes them feel better about themselves. It might not be a very pleasant trait, (and indeed the reason I refuse to watch the audition stages) but it appeals to people on that level.
Wheras with this programme they provide the contrast of the "fatties" who are instructed by what effectively is the grown up version of their childhood bully in a kindly way, and then the "payback" of them being successful in their losses, thus beating the "bully" and proving that if they can do it, so can we. It plays into our brains on various levels, which is why it makes it watchable.