120 Comments and no one has mentioned it.
The large difference to streaming, my guess is that it has to do with a new Video Decoder that fully support VP9 rather than partial hardware acceleration. Previous A14 AV1 and VP9 support on M1 / macOS / iOS all points to higher CPU usage and VP9 is not enabled by default unless you request 4K Content.
So when viewing Youtube, defaulting to VP9 in the same resolution would reduce the data requires to download, as Google uses lower bitrate to achieve perceived quality, this along with some optimisation in 5G Auto Load, better Modem with lower energy requirement ( Expected to be Qualcomm X60 on Samsung 5nm ), variable refresh rate and larger battery.
Note Offline different between 12 and 13 is 40%, that is Battery capacity increase, VRF and new video decoder.