My brother-in-law has kept PepsiCo in business for 30 years drinking the straight surgared original and he’s trim, about your size. I drank diet coke and............you don’t want to know.
I once worked in an office where the fattest guy was a vegan.
I hate to say it now but ... a few years ago I read that articial sweeteners stimulate the appetite. You may not be getting any calories from the diet soda but your increased appetite will cause you to eat more calories from other things than you would have gotten from a regular soda.