Windows 10 has achieved two revolutionary things:
1.It can efficiently compress system files. As a result it gives approximately 1.5GB of storage for 32-bit and 2.6GB of storage for 64-bit Windows.
This is done by compression algorithm that is based on the systems hardware.
2.New recovery method which no longer uses a separate recovery image.
According to How Windows 10 achieves its compact footprint blog article:
– With current builds, Windows can efficiently compress system files. That gives back approximately 1.5GB of storage for 32-bit and 2.6GB of storage for 64-bit Windows. Phones will also be able to use this same efficient compression algorithm and likewise have capacity savings with Windows 10.
– A redesign of Windows’ Refresh and Reset functionalities to no longer use a separate recovery image (often preinstalled by manufacturers today) in order to bring Windows devices back to a pristine state. This reduces Windows’ storage footprint further as the recovery image on typical devices can range in size from 4GB to 12GB, depending on the make and model.