PC = 11, PN = 110, EG = 0.09%

PC = 12, PN = 60, EG = 0.17%

PC = 13, PN = 50, EG = 0.19%

PC = 14, PN = 40, EG = 0.24%

PC = 15, PN = 30, EG = 0.33%

PC = 20, PN = 20, EG = 0.5%

PC = 30, PN = 30*, EG = 0.67%

Here are some math to showcase how important pop capacity modifiers is. Aqueducts are basically useless buildings that you must build to grow cities and the more population you can get out of each of them, the more useful buildings you can build and this will lead to an exponential growth between number of pops and productivity per pop.Since a building need 10 pops, I only looked at cases when aqueducts give more than 10 pops, otherwise there is no exponential growth due to amount of pops since each building slot gained will be taken by an aqueduct. Since an aqueduct provides 4 pop capacity, a +25% pop capacity increase equal to 1 more pop. Two ways to increase pop capacity is horses (+5% pop capacity to provincial capital) and Active trade routes (both import and export in that province), each trade route give +2% pop capacity to all territories in that province. So +25% pop capacity = 5 horses or 12.5 trade routes, to get an idea.The terminology in the Point list belowPC = Pop capacity gained per aqueduct for example 11 mean that your pop capacity increase by 11 per aqueduct.PN = Pops needed to gain 1 useful building, for example a training camp, the rest of pops build slots will have been used up by aqueducts.EG = Efficiency gained per pop, Assuming the useful building give +10%, for example training camp/tax office.* At this level you get two useful buildingsAs can be seen the more pop capacity modifiers your city have, the more efficiency growth a city will have with increase pop and thus its exponential growth will be higher, thus increasing the value of stacking pops in that city.