There is no morale-to-world formula, the developers decide on it when the world is 'designed' and created. It can be changed (they have in the past changed it to make it less difficult). Observation may suggest that larger worlds have higher settings and this may be what they follow, but it's not a direct formula within the game. A large number of the settings are variables that are set rather than being relative to each other. Large worlds can a small number of slots, but obviously the circumference of small worlds does limit the maximum number of slots they can allocate.