A great post over at MacFixIt explains the math about determining a file size -- and how the folks at Apple decided to follow the definitions of a "gigabyte," "kilobyte" and "megabyte" as they are commonly used in English (or, put a different way, just like in the metric system). So, a kilobyte is actually 1,000 bytes, and "officially" has been since 1999. Technically, the word for a 1,024-byte chunk of data is a kibibyte. Having kilo-, mega- and giga- SI prefixes refer to powers-of-10 in almost all realms, and powers-of-2 in information technology, was apparently becoming too confusing.
What does that mean in the real world? MacFixIt sums it up best:
It's not a new issue at all for people dealing with changing clothing sizes, especially for women. What used to be a size 12 back in the 1950s is considered a size 6 today. And a kilobyte weighing in at 1,024 bytes yesterday is now 1,000 bytes today."For all intents and purposes it means absolutely nothing! It does not change anything in how the computer runs, or how efficient it is at storing items on the drive. It has not compressed any of your data or somehow altered it to 'free up' any more space. Rather, it just means that everything will be reported as being slightly larger than [it] used to be, with the amount of difference depending on the prefix being used (the larger the prefix, the greater the percent difference)."
It's worth noting that you will see different file sizes reported when moving items between Snow Leopard and earlier systems, and the amount of free space on removable drives will appear to fluctuate -- but byte for byte, you've got the same amount of space in each case.