No, this is not a joke or just speculation. The Nazis actually planned to make a tank about 100 feet long weighing 1000 tons.
Right here's a picture of the Landkreuzer Ratte, which would have weighed five times more than the Panzer VIII Maus. Not only that, it would have dual artillery barrels. And the artillery would be taken off of a battle cruiser and affixed to the tank.
But it had its drawbacks:
There was one major issue that this tank had. It would have been so large, that it would have been a target to allied fighters and bombers. This would have been such a major issue that the design would have been practically useless.
See also its big cousin: