Well, I just found this and wanted to throw in my theories on the three void creatures and the voidlings from Malzahar.
Kog'Maw- I agree that he is in a young form compared to Cho'Gath and what we know of Kha'Zix, but I do not think he is a larva. I believe he is in a weakened state because he "manifested in Valoran". This could mean that he had to leave part of himself behind in the void, or maybe he did revert to a younger state. I think he was much larger, not as big as Cho'Gath, and was more of a fast running predator that crippled its prey with his acidic spit. His legs would be longer, maybe still as strong as they are now, his arms longer to allow holding moving bones aside, and his mouth about as big as it is now. Basically, a monstrous raptor that could communicate to a small degree with others like him, rather than other species.
Cho'Gath- "Voidborn, creatures so ancient and terrible that they have been removed from history altogether. It is rumored that the Voidborn command vast armies of unspeakable creatures on other worlds, that they were once driven from Runeterra by powerful magic lost to antiquity." This is a very interesting bit of information from his lore. Voidborn were once on Runeterra, but were driven out. More interesting is that they commanded unspeakable creatures. I think Cho'Gath was one of these leaders and has only recently returned to gauge the strength of his enemies before setting his armies loose. As to what I think of him, if I am right, he is much, much older than any of the other champions, but not so old as before the Rune Wars, when I believe magic was at its stronger. Just as his lore says, he can be called one of the firstborn of the Void, having had more than enough time to grew as the rest of the Void creatures began to spring up.
Kha'Zix- When his lore is revealed.
Malzahar's voidlings- I think these are closer to maggots, the tiny flesh-eating insects commonly found in trash that morph into flies. They grow over time, becoming stronger, but then die in a very short time. Yes, they are rather large, but, compared to Cho'Gath, they are insects.