What causes ground beef patties to shrink while cooking?

All meat will shrink in size and weight during cooking. The amount of shrinkage will depend on its fat and moisture content, the temperature at which the meat is cooked, and how long it is cooked. Basically, the higher the cooking temperature, the greater the shrinkage. Cooking ground beef at moderate temperatures will reduce shrinkage and help retain juices and flavor. Overcooking draws out more fat and juices from ground beef, resulting in a dry, less tasty product.