This will make them lose their tone and hence make them flabby. Moreover, not using a muscle long term will make them waste away. This would mean that your metabolic rate will drop, requiring less energy. If you then eat more, this will be stored as fat. And fat accumulates over the thighs and hips in most girls.
Exercise helps to burn up calories from food.
An inactive lifestyle therefore encourages weight gain. But it is untrue that sitting down in particular causes big thighs.