[Chewbacca]

Okay I read the strat guide abit more about stage limits....Basically your overall population effects the size and capacity of the indivual buildings in zones.
In order to increase population density you must grow over-all population by zoning more!
Each kind of zone has its own threshhold of population when its buildings get "bigger". Thats what the graph in the book is all about.

To quote the book- "Zone out in order to zone up." Building more zones actually increases populaton in two different ways.

Ultimately there are a whole variey of factors that effect population.

Also-I found it useful to inspect my zones and re-zone any buildings that had zero occupancy into a zone-type that was in demand. "Dirty" industry can be upgraded, and light residential into medium residnetial will help to.

If you have ALOT of commercial zones that arent developing. change them into residential and see a big boost in population and job demand. Commerrical zones are way more important when your city has 30,000-50,000 population and an airport.