But more energy required to decompress, surely? (Not counting the compression step because I assume that's marginal at scale).

Not necessarily, the decoding can be stopped once you get enough usable information in regards to the usage of the image (display size ...) with the same source image. That's neat!

See the responsive part in https://flif.info/example.html

We can imagine decoding taking into account battery save mode, bandwidth save mode...