If Apple is able to backup my 64 GB iPhone with only 15 GB, and back it up completely, obviously Apple is being extremely efficient with space.
Think about it, if 5 million people own the app angry birds, Apple doesn't need to keep 5 million copies on their server. Apple only needs a list of the apps and songs you have in your phone and then a very small data file to show where you have them and things like top scores and accomplishments. In this way, Apple is able to completely restore your iPhone in the event of data loss by using this method, and do it for some people with less than 5GB of space.
Those that are griping about a free gift of 25 GB don't realize what a blessing this is. I had just cut down my backup from 16 gig 14 gigs to save myself $20 in the coming year, but now I can have my iPhone completely restored with a 16 GB backup, and I save myself $40.
And for those that are wondering why Apple is doing this, obviously they screwed up when they gave MobileMe users an extension through 2050, so in this way they give these people a free year of service, and come out the winners in the PR department.