My own second-favourite way-to-visualise-the-quantity-of-something is that all the gold in the world (not including gold we have yet to dig up or somehow extract from seawater) would make a cube only 20 to 22 metres on a side, depending on who you ask. To help visualise the size of the cube, 21-ish-metres is about the length of two city buses parked nose to tail.
I think the conclusion is that we’re really bad at estimating volume.