If you build up the muscles by working them, they will get bigger. But, it is pretty hard to build them up, especially if you are a girl, I don’t know your gender. It is very good to strengthen your stomach, also known as core muscles. It helps protect your back, plus other good things.
You cannot spot reduce. If you want to lose your gut that mostly has to do with what you eat and the calories you burn to get rid of the fat on your body. Another factor can be if your sugar tends to run high many believe this contributes to fat held in the stomach, you can get a simple sugar blood test while fasting and make sure your level is normal. If you tend to be constipated that can also blow up your tummy a little. And, genetics of course.
Swimming, dancing, running will strengthen your core just by doing those things, without necessarily specifically doing a sit up. Using your legs, epecially going forward with your inner thigh (so foot turned out like a ballerina) engages the somach muscles. Always stand tall, with your stomach held in to help strengthen your stomach and back muscles througout the day. The more you use those muscles the stronger they will become.
More muscle in your body will mean you burn calories faster.