When Snow Leopard came out, a number of people were confused by the sudden increase in free space on their hard drives, but they also were curious about why file sizes seemed to be decreased. This was because in Snow Leopard Apple changed the file size definitions from being based on the binary system to being based on the decimal system (a more logical approach, from some points of view). Despite this change in the Mac, it seems Apple has not included it in portable devices.
According to this knowledgebase document, the only OS to incorporate this change so far is OS X 10.6 and later, so if you have files on iPhones, iPads, or iPod Touches, they may be reported as having a larger number of gigabytes, megabytes, or kilobytes of data than when you transfer them to your Snow Leopard machine; however, if you also have a computer running OS X 10.5 "Leopard" or earlier, the file sizes will be reported as being the same.
We, but to recap, the size differences are nothing but a definition change for the prefix terms used in file sizes. The total number of bytes in the file is still constant; however, what we call a "gigabyte" or a "megabyte" is different on Snow Leopard than it is on Apple's other platforms.
In Snow Leopard, instead of having 1,073,741,824 bytes being called 1 gigabyte as it is defined in the binary system, the same number of bytes will just be called 1.073 gigabytes as it is defined in the decimal system. It only appears to be larger because of the definition change, but is still the same number of bytes, regardless of what device it is on. The reverse is true for a 1GB file on a decimal system being moved to a system calculating sizes based on the binary system. They will appear to be smaller with the binary calculation.
One reason for this change is it is more logical for the ever-expanding nontechnical computer-using crowd, but it also better matches up to how drive manufacturers have for the longest time been advertising drive space on their devices. When you purchase a 500GB drive, that drive size is based on the decimal system. In older operating systems it would appear to have about 465GB (binary gigabytes) in total, but in Snow Leopard the drive now shows up as being very close to the "advertised" 500GB size (actually a little less based on room used by partitioning and formatting).
Sooner or later I expect Apple will migrate the iOS and other devices over to the decimal system, but for now all but Snow Leopard still use the binary system, so be aware that files will appear to be smaller or larger on each system.