Windows backup size

My friend has just got a 500gb USB3 drive for backups and I showed him how to partition it into 3 parts. His laptop has C and E partitions which total about 85gb. When he selected one of the partitions on the backup drive and ran the Windows backup the finished backup was 105gb. How can the backup be bigger than the original? He will be making another backup on another partition to find out if it as the same size or not, but not till next week.

The built in Win 7 Imaging utility includes all partitions it sees as "system" partitions when a backup image is created.
Maybe it is including more partitions than you think it is?

You could try creating a system Backup Image with a free 3rd party program such as Macrium Reflect.
See what size image is created with Macrium.
With Macrium or other Imaging programs you have more control over what is included in the backup image.
You can include or exclude specific partitions very easily.
Here is a tutorial by whs for using Macrium, and it has a link to download Macrium:
Imaging with free Macrium

If you're interested, you could also try mounting the W7 Backup Image as a VHD to see what is in it.
This tutorial by Brink can help with that:
System Image - Extract Files Using Disk Management

I don't use the built in Win 7 Imaging utility because it is not controllable enough for me...

Thanks, I will give Macrium Reflect a try out and will pass on your advice to my friend. I have only used the Windows backup before but have never had a backup that was bigger than the original.

I have just downloaded Macrium Reflect free and made a system image which totalled 68% of C drive (I have only one partition on this pc for W7). I also made the boot disc.