There are a few reasons why your burgers might be shrinking. One possibility is that you're not packing the meat tightly enough when you form the patties. Another possibility is that you're cooking the burgers on too high of heat, which causes them to contract and shrink. Finally, if you add any liquids (like Worcestershire sauce or ketchup) to the ground beef before cooking, this can also cause the burgers to shrink.
To avoid these issues, make sure to pack the ground beef tightly when forming the patties, cook on a moderate temperature (not too high), and don't add any additional liquids. With these tips, your burgers should stay nice and plump!