It is a well-known phenomenon that people attach meaning only to a sum of money they can imagine. For most people it's a sum of money they deal with in everyday life.
So, a person know what lunch money is and what monthly mortgage or rent is. A person pictures how this compares to the price of a house (ususally somewhere in hundreds thousands). By extension, a person can picture what a few millin will be: the price of a few houses.
Anything beyond that is just a number for absolutely most people... even those on the boards of large corporations and in the government.
This explains why so little time is spent by politicians arguing about the size of the bank bailouts or budget deficit or givernment debt. For them, like for most people, these are just abstract numbers.
These vidoes are great for putting it in prospective.