These experiments are mainly just correlational, they do not control the usage of diet sodas. So, of course, people who drink diet sodas are going to be heavie

These experiments are mainly just correlational, they do not control the usage of diet sodas. So, of course, people who drink diet sodas are going to be heavier: they're the ones trying to loose weight and hence the ones who will gain the most weight! There's no reason to drink diet soda if your body doesn't have any problems with weight.