Apps' size on iTunes and size on iPhone 4s varies?

Example:
iTunes has the imdb app listed for 6.8 mb
while
iPhone 4s has the same app listed for 16.8 mb
that 16.8 mb breaks down into
a 9.4 mb app size and
a 7.4 mb "Documents & Data"
Why is this?
Even for apps I havent even used, theyre using up a lot of space.
Thanks!

because the .ipa is unpacked on your iphone very simmlair to zip files