When there is limited resources available, in nature, there is sheer competition and only the fittest can survive. This indicates a logistic growth curve in the population.
If there are unlimited resources, the chances of competitions are rather less. This leads to exponential growth of the population.
In expanding age pyramid of a population, the percent individuals of pre-reproductive age is largest followed by reproductive and post reproductive age groups.
Conversely, in a stable population, the percent individuals of pre-reproductives and reproductive age group are nearly the same.