What’s the purpose of a tank? A tank design is a balance of three factors–firepower, protection, and mobility. You want the most firepower, with the best armor, with the best mobility.
The problem with superheavy tanks is they focus on firepower and to some extent on protection, and sacrifice mobility. You might think that a superheavy tank with superheavy armor would have good protection, but that is only one part of protection. A superheavy tank is a big target–literally, and is extremely vulnerable to mobility kills.
A giant cannon is no good if the AFV can’t move the cannon to the place where it is needed, and can’t protect the cannon from the enemy once it gets there. And of course a giant cannon isn’t any good if you can’t aim the thing and fire it rapidly.
And all this ignores economic or logistical issues–home much does the vehicle cost, what sort of supply chain does it require, how many crewmembers, how prone to breakdown are its components, how easy to repair is it, and so on.
So the point of a superheavy tank is to haul a giant cannon around, and protect the cannon with super thick armor. But that takes a big engine, which means a big chassis, which means everything is bigger, so you need to spread the same amount of armor over a larger area or add even more armor.
In a very real sense modern main battle tanks are already “superheavy” tanks by the standards of WWII. The have giant guns, massive armor, advanced targeting, and good mobility. And the role of medium and light tanks is taken over by other vehicles like Bradleys or Strykers.
So what is the role for an even heavier tank? What is it supposed to fight? Modern MBTs? Right now our MBTs are easily capable of defeating comparable MBTs, you don’t need a bigger and better cannon, the existing MBT weapons are capable of chewing through existing armor like tissue paper. If you really want a heavier cannon then something radical like the https://en.wikipedia.org/wiki/Stridsvagn_103 might work rather than just upscaling a regular tank.
Keeping the same cannon but adding even more armor doesn’t add that much protection, modern MBT armor is as tough as we can make it. You’re just making a bigger target with more opportunities to fail. Making a tank bigger but keeping it light by adding empty space makes no sense, the tank should be as small as possible while still being able to use its weapons. Think about it, if you can eliminate the void spaces you can lower the weight by reducing the surface area of the vehicle, this allows you to have a lower armor weight but keep the same armor thickness. Making a vehicle bigger just to make it bigger serves no purpose.