Too cook the beef, the heat should penetrate to the deepest layer of the meat. Let say that the meat is cubical with 10cm width. To penetrate the core of the meat, the heat need to travel 5 cm from any side of the meat. If the meat cut into two similar pieces, the deepest core of the meat will be exposed to the heat directly.
Now the deepest core is not 5 cm anymore, it will become 2.5 cm. So the heat has to penetrate a thinner layer, thus make it cooked faster.
