Jamieson, Fausset and Brown on Mark 6:42

42. And they did all eat, and were filled--All the four Evangelists mention this: and John (Joh 6:11) adds, "and likewise of the fishes, as much as they would"--to show that vast as was the multitude, and scanty the provisions, the meal to each and all of them was a plentiful one. "When they were filled, He said unto His disciples, Gather up the fragments that remain, that nothing be lost" (Joh 6:12). This was designed to bring out the whole extent of the miracle.