A flattening of the curvature of a geometric progression will come with a decrease in the constant factor. For a graph of population, this decrease will come with a decrease in birthrate or an increase in deaths/decrease in lifespan.
Work Step by Step
Since the point to point change in a geometric progression is due to more organisms, to reduce the change/constant factor think about what causes it. More organisms will cause it, either because more are born or because fewer die. Thus, fewer births or more deaths will result in a lower constant factor.