This is going to be vague, because I'm relying on my shaky memory, but I remember (vaguely!)reading about a study where half of the participants received a certain amount of calories in the form of diet cola, and another group received them in the form of food, in addition to their regular diet. The people who drank the colas ended up consuming more calories overall (they drank diet cola in addition to their regular food intake) while the people who ate the food calories ended up eating fewer overall calories (they ate them in place of some of their regular food).(And doesn't it always seem that the people in the grocery stores who buy diet pop also have a cart loaded with junk food, sugary, fattening pastry, and other empty-calorie foods?)
My personal theory is that your body doesn't register certain things, like diet cola, most "diet" foods, candy and junk foods, as nourishement,(because it contains none) and still craves food. I know I can go through a bag of potato chips with speed and ease, and still not feel satiated, but if I eat a baked potato, I feel satisfied.
As someone else said, diet colas/diet drinks are not at all good for you. As a substitute, try this: a glass of water (boring, I know) with a few drops of stevia extract (liquid) in. Stevia is a natural herb that is extremely sweet. It also doesn't cause a rise in blood sugar the way many other sweeteners do. It's approved by the FDA as a supplement, but not as a food additive (this is basically a political issue, and not a health issue--if stevia were used in food, sugar manufacturers, diet sweetner producers and the like would lose a lot of business). As I said, it's EXTREMELY sweet, so just 3-5 (max) drops of extract in a large glass of water is enough. Go easy at first--if you put in too little, you can always add more, but if you put in too much, it goes beyond sweet into being cloying and weird. Also, I'd steer clear of the powder (unless it's the "sugar packet" like powder that's mixed with something else to cut the concentration), because it's darn hard to get a small enough amount.