Most 1080p bluray rips that I've seen are in the range 30 GB - 45 GB, bit for bit.
Now, downconverted "1080p" videos are much smaller. Maybe as small as 2.5 GB, or say 5 GB, 8 GB.
As I understand it, we have these basic video parameters:
- frame size (1920*1080)
- frame rate
- color depth?
- codec type (h264, h265, etc) compression standard used
Now, sometimes when I open up a "1080p" movie file, the frame size is indeed 1920*1080, and the frame rate is approx. 24fps. This is the same as the bit-for-bit bluray rip.
However, the video itself is definitely not of the same quality - blacks are not deep, some extra grain.
So my question is - what is the information that is lost after doing a lossy downconversion, even though frame size and rates are the same? Somehow is the framesize actually much smaller, with multiple identical pixels being used as fillers? Or is it something else?