The body of every human being fits into one of three broad types. These types are found in every race, in our world and this.
The ectomorphic type tends to great height and a slim frame.
The mesomorph tends to moderate height and a muscular frame.
The endormorph tends to short stature and a portly frame.
The people of this island developed a social structure based around these three types.
The ectomorphs served as priests, philosophers, administrators and the like.
The mesomorphs served as warriors, labourers, and farmers.
The endomorphs served as an emergency food source.
The endomorphs naturally rebelled against their lot, and would often run away (obviously not very quickly). As this spoiled their flavour, resigning the endomorphs to their lot was a major preoccupation of the ectomorphs.
Male endomorphs were at first castrated. This kept them from lustful activity, which is physically draining and stressful, and provokes a preoccupation with appearance, all of which tend to slimming. However this practice was abandoned, in the hope that this concession would satisfy the endomorphs, and distract them from thoughts of escape. It led only to more aggressive and energetic endomorphs.
Endomorphs were then given an increasing series of privileges, such as houses and land. The religion of the island turned towards praising the endomorphs' willingness to sacrifice for the good of the community. As their status rose, the island became more wealthy through trade, meaning that fewer and fewer were eaten.
At the time of writing the endomorphs are now the effective rulers of the place, with the other two body types acting as their servants. Those who are eaten are usually the losers in political struggles, or the worst criminals.
The primary god of the island is the sun, which they call The One Who Goes Forth Shining. They make no roads, nor even corridors in houses, that run directly east to west, that path being reserved for the sun itself. The only exception is certain important temples, and these passages are only walked during ceremonies.
Travel directly west to east is also disdained by the more pious, the sun being thought to travel in that direction through the underworld at night.