That's what most people would think. But an acre is defined as 43,560 square feet (approximately 1/3 of a meter, for everyone else in the world).
So a cubic acre would be . . . . 43,560 cubic square feet.
Cubic.
Square.
Feet.
Cubic square anything would at least be something to the power of two to the power of three!
So that's feet^6.
Considering we are only capable of observing four dimensions at this time (and during the early years of Uncle Scrooge Comics, most people weren't even sure of that many), the Money Bin is incapable of being observed, fully, as we understand it. Which just makes it constantly being at threat of robbery even more inexplicable.