Society level has nothing to do with population or production.
from the help pages
Society Level
Production of resources on a planet are controlled by it's Society Level and population. Surplus resources are produced if the people on a planet are able to produce more than they use during a turn.
Unless your planet has suffered population loss from an attack, society level and population will be reasonably linked. For example from my colonies
Society Level 16 Population 2468
Society Level 17 Population 2763
Society Level 18 Population 3065
Production is a function of the two, a planet with a higher society level will produce more then one with a lower level given the same population.
