Ultra-HD discs ship with video in HEVC, so they'll benefit the least from a potential re-encoding to AV1 for space savings despite being the largest space-takers because they ship four times as many pixels as HD :D
For MPEG-2, VC-1 and H.264 discs I can get much more significant space savings if I decide to re-encode them, but I have to deinterlace any 480i, 576i, or 1080i material.
Interestingly a lot of Blu-rays these days ship lossless audio tracks which are also quite large!