Why do my burgers end up round? When I broil or grill my burgers, I end up with burgers that are round in the middle. 

I grind my own beef blend and form my patties the same every time. When I cook my burgers in a pan I don't get the rounded burgers. 

What makes the burgers rounded and smaller when broiling/grilling, but not with pan cooking?
I'm not sure what causes the bowing to be more pronounced from one cooking method versus another, but the solution is to put a thumbprint indentation in the middle, creating a slight "donutted" shape.   This will help to ensure you get the "patty" shape you want.