There are all sorts of things not said in this that could matter.
First of all, younger people are far less prone to having reasons to visit their GP. As a group they weigh less. But there was no mention of the relative ages in the study.
Second, the human body is designed to put on and retain weight, to be less susceptible to starvation. Therefore being somewhat overweight as you get older is normal. If you are of normal weight, or underweight, it may be indicative of serious health problems.
Third, the diseases experienced by the obese are generally chronic, compared to those who are underweight, who tend towards acute and deadly problems.
However, those who have acute diseases and die cost far less to a health care system than those who need to care for their diseases for years.
I see obese young people riding buggies with loads of junk food at Walmart and the older thinner shoppers are walking and have fruits and veggies!.