Economy in Gal Civ II was based on pop. More pop = more credits that can be CONVERTED to something else. Since pop grows linearly, that meant economic growth was linear and predictable. And relatively easy to control.
The factories and labs in Gal Civ II actually provided ZERO ECONOMIC BENEFITS. What they did instead was they CONVERTED the economic credits from POPULATION to something else like starships or research. THIS IS FUNDAMENTALLY VERY DIFFERENT FROM THE GAL CIV III ECONOMIC MODEL.
Economy in Gal Civ III is based on ADJACENCY BONUSES AND STARBASES AND THE MASSIVE % BONUSES THEY PROVIDE. WHICH SCALE EXPONENTIALLY WITH GROWING POPULATION. In other words, Gal Civ III's economy is based on EXPONENTIAL GROWTH.
See the problem?