Both of these hernia may allow some abdominal contents to exit the abdominal cavity. Repair may actually increase abdominal volume. With a n umbilical hernia, if there is a large bulge that would be gone and may improve the contour of the abdomen. All the best.
Hiatal hernia does not increase the size of stomach. A general surgeon will repair the hiatal hernia.Unless you have a large umbilical hernia, it will not decrease your stomach (abdominal) size.If your concern is abdominal contour, please consult a board-certified plastic surgeon to learn about your options.