Microsoft is working on a new edition of Windows 10 which internally calls 'Windows 10 Lean'. It could be considered as Windows 10 on diet, once installed, is a whole 2GB's smaller in size compared to a normal edition of Windows 10.
Why is Microsoft building on Windows 10 Lean? Microsoft is facing a problem when it comes to keeping devices with a low amount of storage up to date with the latest Windows10 feature updates. Usually, cheap, low-end tablets and laptops with 16GB of internal storage will find themselves stuck on the version of Windows 10 that those devices shipped with, because there's not enough storage space to apply any new feature updates. It is important for Microsoft to ensure that these devices can install new versions of Windows 10.
Windows 10 Lean aims to fix this problem. For a device with a small of storage, it strips out things users won't need, such as the Registry Editor, Internet Explorer etc. It's still the same old Windows system, just streamlined so that it better fits on devices with smaller internal storage. Windows 10 Lean does not remove support for legacy apps, meaning it still has full Win32 app support if you need it. Other than the differences mentioned above, it's still legacy Windows at the end of the day.
However, Windows 10 Lean is very buggy in the latest Redstone 5 builds, and still in development internally.