Nerves is around the 10-20 MB range. The main downside of the Grisp system I could see would be that nerves has a really nice two phase firmware update story, and that isn't mentioned in Grisp that I've found. IMHO, the remote firmware update and tooling is a "killer" feature of nerves.
Firmware update in the field is important but only a small aspect of a embedded system (one that really needs to work).
But Erlang already offers a lot around releases out of the box and in our case The VM + the whole "OS" is just one file to swap so we don't have a lot of problems one has if Linux + Erlang + Application needs to be updated.
FWIW: for larger Embedded systems we use FreeBSD + nanobsd wich solves a lot of the above mentioned problems. Update image verification and running the update is about 200 lines of Erlang code. And nanobsd comes with the normal FreeBSD system (builds disk/flash filesystem images with multiple partitions for easy update -- runs from one swap to the other)
