I just have to say, what a bunch of beautiful cats in the photos in this thread. Cartman and Ronnie and Nekochan's cats.
Cartman ( who has a pattern called Brown Mackerel Tabby and White) , to me actually in some ways reminds me a little of some of the foundation cats of the Norwegian Forest Cat breed. Not that I'm saying he's part that breed, but as Oriental Slave says, the 3 Northern "forest cat" breeds -- the NFC, Maine Coon and Siberian were all developed from "natural breeds" (and it's possible that they are distantly related ) , and in any case, there are still a lot of random-bred cats who do resemble these breeds in some way. though with unknown ancestry one can't really say they're the breed. ( but if a cat very much resembled the breed AND came from the place of the breed's origin, then one could call it a native cat of that breed ( and at least in some registries, NFCs and Siberians from the area of origin can still be used in breeding programs.)
Ronnie , the Cream Classic Tabby, IMO does not strongly resemble any of those breeds, but has some characteristics of each of them. The very long body is more like a MC but he doesn't otherwise resemble them. and the head looks somewhere between the other 2 breeds.
They are just their own unique selves.
You can't really tell by size . A relative of mine had some huge Domestic Longhairs in the 1980s and I'm sure these days with Cats 101 someone would be telling her they must be Maine Coons. (LOL, and especially the tabby since he had the M.... I still can't believe Cats 101 made such an idiotic error as to say one of the most common markings in cats was a sign a cat is a specific breed ) but they were just random- bred rural cats and she was nowhere near Maine.
Also with weight, people often quote their cat's weight, thinking it might be a sign they are a Maine Coon, or Ragdoll , but 1) domestics do come in a wide variety of sizes, and 2) often the cat is several pounds over what their ideal weight would be so the weight is even less significant.