The moderately overweight have the statistically longest life span, interestingly.
Most commercial dieting programs (and they generally work pretty well today, to their credit) use old ideal weight tables that are about 40 lbs. too pessimistic in terms of ideal longevity. Now some of the bias away from the slender has been said to come from tobacco smoking (smoking cuts the appetite but easily causes lung injury and disease too). But fewer and fewer people smoke today and it seems to be still true.
Our species evolved to survive regular cycles of famine. If you didn’t pack on a few extra pounds, whenever there was plenty of food; you wouldn’t survive the next famine. Body fat is now considered an organ, as it produces the hormone leptin. When we lose too much fat, the remaining fat screams at our body to eat more. For many thousands of years, this was essential to survival. Now, when few in the western world ever know hunger, let alone famine, these survival mechanisms make it hard to keep our weight within an “acceptable” range.
The “ideal” weight tables seem to have more to do with aesthetics than they do with real health requirements. Interestingly, the aesthetic standards have changed dramatically over time. When food shortages, and periodic famine was the norm, plump people were considered most attractive. In men, the extra pounds was a display of wealth. In women, the extra pounds made it more likely the woman could bear and nurse healthy children. Only the poor were skinny. How times have changed!