I put in a new habitat (version 6.0.62) and when it leveled to 5, the number of max citizens is at 16.
But the ones put in at version 6.0.58 or earlier was 18.
I could not find any notes about this change.
Yes, and well observed 
Daniel did make a comment about it, somewhere 
Just cannot easily find it atm.
fwir it was just a game balancing thing 
Found the note about habitats.
It is at:
Friday Dev News #20 - Upgrades
section - Balancing, other changes
item - habitats 3-6 don’t hold as many people anymore
I did not notice until now.