Grilled burgers tend to bulge in the center as they cook because their edges contract from the heat, leaving the center nowhere to go but up. They often look misshapen and cook up smaller than the bun, and the thicker center takes much longer to cook than the edges, which can become well done and dry in the meantime. Rather than flattening the burgers with a spatula, which presses out all their lovely juice, try forming your burgers a little differently. Begin by making a flat patty that's 1/2 to 1 inch wider than the bun, since shrinkage is inevitable. Next, make an indentation in the middle of the patty with several fingers so that the top is concave. This way, when the center bulges, it will end up being about the same thickness as the edges and the burger will cook more evenly.