I go to the gym regularly to do weight training and start off with sit ups to warm up, however I never seem to lose my belly. I'm not interested in a six pack I just want to know if training your stomach muscles can actually increase waist line.
I answered a similar question to this and went into great detail on "what to do to build a six-pack." Now, while you said that isn't your end goal, I still recommend reading through that answer because it does a great job of explaining your problem.
Now, without overlapping too much on the content from my earlier answer, let me explain exactly why you aren't seeing your desired results and what you can do going forward.
I go to the gym regularly to do weight training and start off with sit ups to warm up, however I never seem to lose my belly.
This is because your waistline is defined by the layer of fat on the outside, not the strength of the muscles on the inside. Doing ab exercises strengthens your abdominal muscles, but it does not burn the fat surrounding said muscles. You could conceivably have the strongest abs in the world, but if you don't have a low enough body fat percentage (BF%) then it will just look like you have a beer gut. The takeaway here is that ab exercises are not effective at reducing your waistline.
What can you do instead to reduce your waistline? Well, as Dave already said, there is no such thing as spot reduction. This means that your only solution for losing fat in a certain area is to just exercise and eat well until your body decides to burn the fat from that area. Unfortunately for you and everyone else, the stomach is one of the last places the body will pull fat from, which means you have to stick to a diet/exercise routine for a while before your body will start burning away that fat.
I'm not interested in a six pack I just want to know if training your stomach muscles can actually increase waist line.
As you now know, strengthening muscles does nothing to reduce your waistline; but can it possibly increase it? The answer is no, but also yes...
I said no because an abdominal exercise does nothing in and of itself to notably increase your waistline. On the other hand, I answered yes because your waistline can increase as a result of following a proper strength training program. This is because in order to see results you need consume an excess of calories to properly fuel muscle growth. A side effect of this calorie surplus is that the excess of what you do not use for fuel will be sent to your fat reserves, thereby increasing your waistline.
Because the goals of strength training and fat loss are so at odds with each other, the typical approach is to go through cycles where you focus on strength training for a period, then you go through a cycle of "cutting" where you switch your focus to burning fat instead of building muscle.
Posture can contribute to a belly as well. If your chest muscles are over developed compared to your back muscles (which is quite possible if you're doing lots of crunches and sit-ups, and other popular core work), then your shoulders will be hunched forwards. So addressing your shoulder and upper back muscles is quite likely key, and possibly also your lower back muscles.
I go to the gym regularly, but my fitness trainer says that crunches are must even if you got a pot belly. I questioned, whether it won't effect the burning of fat layer on my belly, he answered plain no. Infact he said, crunches will assist burning of fat layer. Now I know that he was wrong, what should I do now?