I did a backup the other day of a brand new Lenovo laptop that shipped with Windows 10 and that worked out at 27GB. From memory, with a fresh install of Windows 10 on it, the HDD was showing under 20GB used in total. Assuming Windows Backup uses some sort of compression, that's quite a difference.
Pretty sure Windows 10 is actually quite small (certainly comparable with Win7 that was 16GB for 32bit and 20GB for 64bit if I remember correctly). I think Azza is on the right track - it's not what Microsoft put into Windows 10 that's the issue, it's often the extra add-ons that the manufacturers put onto new builds that is the issue. This is why the first thing I recommend anyone do with a pre-built laptop/PC is to wipe the whole drive and install Windows yourself to get rid of all the extra junk nobody needs.