I remember a trick from the early web days of halving the resolution of a JPEG and then expanding the compressed result back to the original size. It'll be blocky, but much less blocky than just reducing the JPEG compression level for the same fize size reduction at the low end.

I believe WebM does (or at least the format can do) this automatically in low bitrate situations, without the external user needing to understand what's happening on the encode or decode end. I don't know if WebP inherits this trick and that explains it's low bitrate performance or if it's just better in other ways.