I discovered that this open format is supported for upload and storage on this forum (probably due to Discourse allowing it). All the major browsers support it. It compresses better than JPEG, typically 20-25% smaller. Here are two images first in JPG:
Then in AVIF:
Real-world images also work well in case you think that this image is atypical.
https://aomediacodec.github.io/av1-avif/latest-approved.html
Webp has also become popular over the internet. I can open webp with gimp, but not with my regular “picture viewer” on my linux box (with an old Linux Distro, badly in need of an upgrade), and that’s a bit of a nuisance for me. AVIF don’t show at all, (It probably would if I put some effort into finding some plugin or other software). Some formats are also burdened with the royalty thing. I guess there is no problem for viewers, but you have to pay royalties for editors. That is a big problem for open source software, as there generally is no budget for that.
Size of JPG is also very much dependent of the compression ratio.
Your pictures are 192KiB for the JPEG variant, and 149KiB for the AVIF.
Here is your jpg, but compressed to 100KiB:
And to 50KiB. With this last one compression artifacts are becoming quite noticeable.
When doing any reasonable comparison of lossy compression formats, you have to take the amount of compression artifacts into account. And that also highly depends on the sort of picture that is being compressed.
Did you
a) start with a lossless image, and compress to jpeg and to avif, or
b) start with a jpeg and recompress to avif?
I assume the latter, and that your starting point was
o-rly-collection/public/book_covers at main · denitdao/o-rly-collection · GitHub
That looks normal to me these days. 
Not very impressed. That bird’s cover is 500x700 and still 39KiB
Below the original upload of the jpg, but re-compressed at the maximum that Gimp allows. Still 912x1200 pixels, and 27.5KiB in size.
Compression can be done further, (with more quality loss), but this is the maximum that Gimp allows (without trying really hard).
Edit, oopsie. After the upload, this forum’s software recompressed it again to 26.8KiB, see below.
It was meant in jest - the original images linked in @retiredfeline are spoof covers that resemble O’Reilly technical manuals but with a humerous twist; in this case a reference manual on compression with an appalling example of over compression artefacts. The link provided has many further amusing examples.
