“Getting the munchies” is a phenomenon that occurs when the production of ghrelin is stimulated in a way that is independent from the normal cycle of metabolic hunger. Ghrelin is a hormone that is released when the stomach is empty and whose production stops when the stomach expands (unless you’re smoking marijuana.) It is this hormone that causes the sensation of feeling “hungry”. The inhalation of smoke from the cannabis plant is associated with the release of ghrelin, but scientifically, as far as my research indicates, we haven’t even gotten as far as conclusively demonstrating that it is THC which causes these effects.
What is relatively certain is that a given strain’s effect on hunger will vary dramatically within a substantial population. Furthermore, as people use cannabis regularly… like all the time, lots of different kinds of ghrelin release schedules get developed. One reason why there is a prominent minority of heavy cannabis users who are very thin may be that constant stimulation of ghrelin is something that the body learns to ignore, and in many of these cases, the user’s appetite and hunger cycle is disrupted for the duration of their use.
I personally think that the variations in hunger-stimulation from cannabis strains is more about the human’s phenotype and history of cannabinoid use than the plant’s phenotype.