I've thought about this a fair amount, not in terms of obesity, but simply in terms of mismatch in sizes. I'm really small, and most of my dive buddies are men who outweigh me by close to a factor of two. I know from Rescue class that there is no way I'm going to be able to drag any of them clear out of the water in an emergency. But I CAN get them clear enough to do CPR and have their faces out of water, and that's what matters most. From there, I'm going to need help. I've brought this up with at least one of my buddies, and he's not terribly worried about it, since we almost always dive where there are lots of other people. And it is what it is, anyway. I'm not going to get any bigger, or a lot stronger, no matter what I do.
So in terms of rescue, I think the obese diver is simply taking a somewhat greater risk than a small person, in terms of how effective others will be in rescue efforts.
In terms of heart attack risk, obesity IS a risk factor for heart disease, but it is not the only one. Smoking is probably even MORE of a risk factor, but the worst one of all is bad genetics. If you argue that obese people shouldn't dive because they are at increased risk for the thing that seems to kill the most divers, you really should extend that argument to smokers and people with bad family histories as well. It starts to get absurd very quickly.
It makes sense to keep your body weight under control from a ton of standpoints -- heart disease, diabetes, pulmonary reserve, joint trauma among them. But there are plenty of skinny people having heart attacks, and there will be as long as there are skinny people in the world (something which is, she says as an ER doc, not a given any more!)