From the "Horses" thread...
Here's a theory - Greatshell carapace strength (and by extension maximum phyiscal size) is determined by the capacity of the individual's gemheart (i.e. natural fabrial).
Thus, the spren that emerge from a dying greatshell escape the gemheart at the moment of death are the physical manifestation of the Shadesmar concept {improbably big crab thing that is alive and does not crush under its own weight}. When the greatshell is alive, these spren are attracted to and trapped inside the gemheart. The spren make it possible for their host to breathe Stormlight which not only solves the oxygen-CO2 exchange problem, but also allows for great feats of strength. (We see similar effects as a result of Kaladin's bond with an honorspren.) This allows the greatshell to grow to Chasmfiend size. When the greatshell's connection to the cognitive realm is severed by death, the spren are no longer bound. Their departure leaves the gems inert and able to be cut by an autofabrian to target other varieties of spren.
Greatshells exist throughout Roshar (if I recall), but the ones in the Shattered Plains are particularly massive. Something about the nature of spren or Highstorms in that region (perhaps as a result and/or cause of the final battle of the last Desolation) makes them able to grow much more effective gemhearts.
Chulls would have comparably smaller gemhearts compared to Greatshells, which explains their more docile nature.
At the other end of this growth spectrum would lie Thunderclasts, which have super-efficient gemhearts that can hold/metabolize Stormlight much more efficiently.
To go even further out on a limb... Parshendi would be parshmen that use gemhearts as natural fabrials to maintain a level of shared consciousness. Their fabrials are tuned in pairs, much like spanreeds and are operated using vocal harmonics (i.e. songs) to evoke cymatic resonance. At the far end of this spectrum, you get Voidbringers, which hold Stormlight perfectly and have all sorts of crazy powers.
