That does not necessarily mean that the whole game is indeed in there. While the file size could indicate that parts of the game are just commented out and indeed on your hard drive, it could also be that the dev did not go through the ordeal to check which textures, voicelines, what have you are in the demo and which are not and thus just packed'em all in while deleting the code from it. That way the game would be almost as big but lack much of the content.
A third possibility is that the game uses almost all of its assets in the demo but does not deliver all the content. If for example a game has four characters and all of them are in the demo as well as in the main game, the demo needs almost the same files as the main game.