it would look more impressive vertically stacked... it didn't make me think that one trillion is particularly huge.
Imagine that full of pallets of $100 bills.
To me that's quite a visual, vertical has no spacial reference.
UPDATE: I did some quick calculations and if my math is right, it's more like over THREE acres for a single stacked pallet. Double stacked as in the image would cut it down to just over 1.5 acres.
To me, large amounts of land are definitely more distinctive than vertical, especially when it takes you a long amount of time to walk across, you get the perspective of your small size in comparison.
Let's say the stack was twice as high as the empire state building, does that really give you perspective? To me that would be meaningless, I have no understanding of such height.