Mimas is the larger of the two and the crater Herschel is plainly visible. If you think Herschel is large you would be right, Mimas is 396 km (246 miles) across and Hershel spans 130 km (81 miles) nearly 33 percent, the crater is also about 10 km (6 miles) deep.
Mimas has another distinction: as far as I know and probably owing to its icy nature, Mimas is the smallest object known to be in hydrostatic equlibrium. Briefly stated another way it is the smallest object able to use its own gravity to pull itself into a spherical shape. A note here, the composition means a lot becuase the largest object known NOT to be in hydrostatic equlibrium is the sort of oblong asteroid Pallas at 532 km, but keep in mind it is made of rock to a large degree.
The smaller moon in the picture is Pandora, which really does look potato shaped when paired with Mimas. Pandora is roughly 114 km x 84 km x 62 km (71 miles x 52 miles x 39 miles).
Source: Two Moons Passing in the Night