That's not a toy tank sitting in this thing's shadow. The shells were 11 feet-long, and nearly three feet wide. They weighed 14,000 pounds. The Gustav could hurl them 23 miles. It took a half hour just to load it.
If you're wondering why the war didn't end the moment they rolled this terror onto the battlefield, you have to realize how laughably impractical the thing was. It took 250 men to get it assembled and ready to fire, then another 2,500 to lay twin rails for the damn thing just so they could move it towards Russia, the only country on Earth large enough for Heavy Gustav to hit.
It was basically a weaponized suspension bridge.
The Germans actually tried to fit Gustav's 800 millimeter gun onto a tank, the aptly-named Landkreuzer P. 1500 Monster (yes, Monster). Fortunately, it never left the drawing board of a man who we suspect had a very small penis.