You might wonder why they planned such large tanks, perhaps you should look more at basic human feelings and the generation that spawned all this.
Coming out of WW1 we saw Big Bertha, just the idea that the Germans had a weapon that could shoot the frontlines from so far away must have put the jitters into the allied soldiers.
Imagine then the fear that a tank, that huge could appear on the battlefield and decimate the entire opposition, it may have been a one off tank, but it's psychological effect has far reaching effect.
It's about fear. Kind of ironic that the universes smallest piece of matter, was then to end the whole war with it's fear.
In my opinion, and this is my opinion not me asking to be flamed for 'forgetting those nations that...' The German military is still and hopefully for a long time will remain the most formiddable opponent that Britain has ever faced. Politics aside, the german soldier was the most resourceful, efficient, well-trained tool on the battlefield, such as shame we had to call them the enemy.