about the ratio "saved"
Even though comparing the size of two lossy compressed images is totally meaningless if you do not provide that both have the same viaual quality or at least something like a SNR ratio ...hell, even compressing one using the other as source is totally meaningless.
Anyway, the ratio of 40% sounds not too far off. If you'd took out-of the box h.264 and applied all the tools applicable to still images, you'd probalby end up with a similar figure compared to JPEG, at the same "computed visual quality". No surprise there ... It's almost 20 years since JPEG was born (well also almost 10 since h.264, but still 10 difference...).
Looking at JPEG2000, it also claims around that marging to JPEG. Frankly, i think it performs worse than JPEG at high quality. And while details are somehow better preserved at qualities you really don't want your eyes to suffer through, it looks like shit there. IOW, it is technically a worthy progress and may have some merit at the lowest quality levels, but no thanks.
Now, this web-P ...it's derived from VP-8 .. aren't On2's video codecs based on ..wait.. wavelets, like, um, JPEG 2000? Regardless of whether that is acually true or just a misunderstanding of mine, On2 videos damn well look like having the exacly same crappy degradation performance as wavelet compressed images do. Maybe it is technically not exactly wavelet but its failure mode looks a lot more like typicaly wavelet than typical block-transform. It just doesn't look pleasant.
I hope "40%" is just not enough. While h.264 got away with a "50% off MPEG-2" - basically because the industry needed some new video format to sell "HD", i hope google doesn't yet have the power to force this on everyone. I mean, i perfectly recognize JPEG is old and by now world and dog could do "better" compression wise, but just because google says "40%" i don't think it's worth it. After all, JPEG-2000 faceplanted with wavelets; if it wasn't for youtube/flash crap-league videos, nobody would even know how a wavelet-encoded video looks; h.264 isn't wavelet even though the tech "existed" at the time and the next MPEG codec doesn't smell like it were based on wavelets either. To me, hints at "wavelet compression isn't ready for humans to look at".
Sorry for the tirade. I like the wavelet concept technically (eg for computer image analysis like feature recognition), but my eyes don't like waveletted images at all.