I’m not sure it’s practical. If one is in a large group and doesn’t make use of the insurance, then one has a degree of anonymity in that regard. Unless, of course, you want corporations to have health police who run around with portable scales and calipers and tape measures when someone merely signs up for a plan.
Why not DNA tests or, at least, family histories? Your father died of a stroke at age 47? Let’s raise your premiums 150%.
I look at it this way: one of my co-workers, at age 42, got breast cancer. She has been treated and, thankfully, has been cancer free for two years. But she has to get a number of expensive tests every year and visit an oncologist and a radiologist, in addition to her primary care physician. This is costly. Should we raise her rates? After all, is she at increased risk for a recurrence or, worse yet, having the cancer metastasize (some lymph nodes were involved). Btw, she’s a non-smoker and doesn’t have an ounce of excess fat on her body.
Regardless of her weight or personal habits, she is an expense. Additionally, she has two children who seem to have frequent visits to the pediatrician. One is on ritalin, but other than that all those trips to the doctor, with strep tests and mono tests and prescriptions, mostly for common childhood diseases, cost money. This family is a real drain on the health insurance ratings.
Exactly! Cancer patients are expensive. Right now. Childbearing patients are expensive. Right now. Obese patients might or might not be expensive in the now and in the future. An otherwise healthy person who is significantly overweight and gainfully employed is a net gain to the system.
But here we overlook the group insurance system, which estimates costs across the entire population of its users. That’s the basis for the premiums. In some years some people will use it more than others.
But using health care is expensive; not using it is not expensive. That applies whether one is obese (we could use a definition here) or not. 
After all, if you’re obese, if you drink to much, if you smoke (or chew) or even use illegal/unregulated drugs, you get to a point where you get tired of going to the doctor to be lectured. Unless one is a complete masochist, they avoid going to a physician as much as possible.
Well that’s a good possibility; the laws of unintended consequences. But at least you’re thinking ahead, instead of thinking reflexively.
By the same token, it does presume that obese folks require an inordinate amount of medical care all the time. I suspect that, like many other so-called vices, most health issues don’t present until later in life unless there are specific genetic predispositions to the contrary.
Hey, lots of us do that now and then. 