Oh, and you forgot the Olliphent. It's seen in ODST. It's a smaller version of the Elephant, it's the one that you see at the end of ODST, the garbage truck. It is actually used by the U.N.S.C, but I forgot what it was used for. But it was used for a different reason then the Elephant.
Actually, one of the biggest traits that the Olliphent has then the Elephant, is the fact that it is a little shorter, and a little thinner, so it can fit through areas that the Elephant can only fit the first part of it's self if it backed into the area. And plus because of the Olliephent's size, it can drive extremely faster then the Elephant. But, to make up for the Olliphent's size, and weight, (which makes the Olliphent weaker.) The Olliphent's back Is completely covered up. This makes the inside off the Olliphent less exposed.
But to get me wrong. The Olliphent isn't a replacement to the Elephant, nor is the Olliphent a successor, the Olliphent is simply a different variation of the Elephant. And this variation of the Elephant is just used in areas where the Elephant would be useful, but is impossible to use.
the Halcyon, for it's size, is actually the smallest ship to be given the Cruiser designation: It's barely larger than a destroyer. they were prohibitively expensive, though VERY well built. the Marathon was essentially the attempt to get the same toughness in a larger yet less expensive package. now, if you want a large ship, the Phoenix from Halo Wars qualifies: it's a modified Colony ship, turned into a Carrier-battleship.