Several things can cause big stomach in females such as things like a poor diet, lack of exercise, and stress.
Menopause and a low protein diet can also cause a big stomach in females and cause belly fat.
You can help reduce your big stomach or belly fat by doing things such as Improving nutrition, increasing activity, exercising regularly, reducing stress, and making other lifestyle changes can all help you lose the unwanted big stomach and belly fat.
Some people are also just naturally obese as well and there's not much you can do but exercising and changing your diet can help to lower your weight and lose belly fat as well.
Swimming, rowing, walking, jogging, using a treadmill, exercise bike etc are all good ways to lose belly fat.
Losing belly fat won't happen overnight but with enough exercise, change in diet and determination you can lose the belly fat easily.
Another option is weight loss surgery as well but that is expensive and you have to pay for weight loss surgery out of pocket.