Sky q 4k

Hi,

Wasn’t sure where to post this question but here seemed right place.

I had sky q installed today. I decided to checkout the Martian 4k from sky. I noticed the movie was only 18gb. This got me wondering how the movie is stored/compressed. When I ripped my bluray (1080p) without compression it was about 40gb. Considering the 4k is 4 times the size raw it got me wondering just how good the 4k from sky would actually be.

Any feedback would be cool.