I've recently made hard drive image backups of school PCs. One set of PCs had 500 GB drives and the other 1TB hard drives. Both sets had about the same amount of hard drive space in use...or about 40-50 GB. Creating the image of the 500 GB drives required less than 15 minutes but the image creation of the 1 TB drives took over 2 hours. As well, the image created of the larger drives was about 3X the size of that of the smaller drives. Why?. The answer seems to be that imaging utility (Acronis True Image) defaults to the sector by sector mode of operation at some drive size threshold. This means that, rather than ignoring the free space, it copies that as well.
My version of the utility (2010) is not current, however. That being said, anyone who uses drive imaging in their backup scheme might want to take note of this finding. Personally, I think it's foolish to buy a larger drive just because the cost per GB is less. Who buys a 20 room house but only has 2 rooms of furniture for it?