I don't have the link anymore, but there was one study that looked at mortality over some fairly long time span, based on how many of a set of unhealthy factors you had, versus weight category. Smoking, heavy drinking, not eating vegetables (I don't remember if it also had lack of exercise? Maybe). What it very clearly showed was that the unhealthy aspects multiplied. If you had none of those unhealthy factors or just one, your size category didn't really change the mortality numbers. If you had all of them or all but one of them, everyone's mortality was a lot higher, but you also saw a very clear increase by size category. Basically your grandma is a good example of this, if you are otherwise basically healthy and have healthy habits, being fat doesn't seem to cause a lot of issues. But as you start adding other issues it compounds the effect.My grandma is fat too; although she diets and exercises she's never successfully lost weight. She does, however, have a completely clean bill of health. Probably because she drinks very little alcohol and has been smoke-free for the last 20 years.