This passage comes from page 10 of the translation of Thomas Piketty's book Capital in the Twenty First Century:
If the rates of population and productivity growth are relatively low, then accumulated wealth naturally takes on considerable importance, especially if it grows to extreme proportions and becomes socially destabilizing.
Why does low population growth exacerbate the effect of accumulated wealth?
Intuitively, I would think that an increasing population would mean more potential employees, thereby allowing already wealthy people to more easily offer low wages. By contrast, I would intuitively think that low population growth, and the resultant scarcity in employees, would force the wealthy individuals to offer higher wages.