So they have updated the Kickstarter page to include PC min specs - most noteworthy has to be the 100 GB of HDD space required!
Reading on IGN they noted that the Witcher 3 requires a mere 45 GB of space. (Just as a way of reference)
Minimum:OS: Windows 7x64, Windows 8x64, Windows 10x64 (64-bit OS Required)
Processor: Intel Core i5-4460 (3.40 GHz) or better; Quad-core or better
Memory: 4 GB RAM
Graphics: NVIDIA GeForce GTX 650 Ti or better (DirectX 11 card & VRAM 2GB Required)
DirectX: Version 11
Network: Broadband internet connection
Storage: 100 GB available space
Now I know it doesn't really mean a whole lot but it does point to a large game with a lot of features and details to require that amount of space. Especially as they are using the Unreal engine there will be optimised features in that suite of tools to minimise the amount of HDD space required (I assume).