For some animated videos, the difference was quite impressive to me.

    AV1    100 MiB
    H.265  500 MiB
    H.264  1.5 GiB

At this point, the audio codecs and track count start making a difference, so this isn't really a fair comparison. And BTW, in terms of video quality in the files above, AV1 > H.265 > H.264.

In non-animated content, the difference is less impressive, but comes at about ~20% in favor of AV1 vs HEVC, in my subjective-and-not-rigorously-benchmarked opinion. But "video quality" is subjective anyway.