At large population and non-zero colony cost, adding population reduces the absolute (not just percent) number of available workers. Not sure if this is intended or not.
Here are before --> after stats for Mars (colony cost two). Before is at the last econ update; after is now:
infrastructure: 56946 --> 56896 (LOWER)
total pop 256.33 --> 256.68 (higher)
ag+env 38.45 --> 38.50
service 182.38 --> 182.71
mfg 35.50 --> 35.47 (LOWER)
This took me from having 0.15 -->0.12 free workers. Note that colony cost is 2.0
I suspect that what's going on is that colony cost is adding a flat percentage to the ag+env percentage (looks like it's 5%*colony_cost - I thought this was workers being assigned to infrastructure in another post). As ag and service go up due to population, this flat (population independent) portion eats more and more of the available workers. Finally, when the population level reaches a point where the mfg percentage on a cost==0 world would be the env percentage, the env percentage eats all the available mfg workers. So for a colony cost 2.0 world, when the percentage of mfg workers would normally be 10%, the number of available workers is zero (because the 10% are devoted to env work).
If this is not what was intended, then a way around it would be to have the ag+env be an absolute percentage, and the service/manufacturing sectors divide up whatever remains of the pie, In this case, a population that would have 20% ag, 60% service, and 20% manufacturing on a cost=0 world would have a 75% to 25% services to mfg mix (60% is 75% of the 80% left over after the 20% of ag are subtracted). If it were a cost 4 world (env 20%), then the breakdown would be 40% ag, 45% service and 15% manufacturing using the new formula (45% is 75% of the 60% left over after the 40% ag+env is subtracted). With the old formula the numbers would be 40% ag+env, 60% services, and 0% manufacture. Note that I cooked the cost=0 numbers and chose cost=4 for the example to make the fractions work out nicely

John