I have a backup of Windows 8.1 that is around 30GB+ in size. I also have a Windows 10 backup that is around 10GB in size. Why would there be such a huge difference in size? Please keep in mind that I have all my Music, Videos, Pictures, Documents and Downloads going to my 1TB hard drive, while I have a 120GB SSD as my boot drive which is the one I have backups of. Other than being different OS, and the abundance of Windows Updates for 8.1 compared to Windows 10, and all the programs are the exact same, is there something I'm missing about the size difference? Both Backups were taken at the same point, once all programs are updated to their latest version and all Windows updates installed. This is my usual setup for Windows 8.1

Diskpart Clean SSD
Recover Backup
Install all Windows Updates
Install all Program Updates
Delete Previous Backup
Do New Backup
(This is done every 3-4 months)

So far on Windows 10, all I've done is first backup, installed all programs and Windows update.