= Solution
For fixed epoch and the increasing branch $\nu>1$, increasing mass lowers the <smoothed matter density variance>, hence raises the <halo peak height> $\nu$. The rate $(\nu^2-1)\dot D/D$ therefore increases and its inverse decreases. \b[Rarer, more massive haloes have a shorter fractional abundance-growth time], even though their actual number density is much smaller. They lie farther into the exponential tail of the <Press-Schechter halo mass function>, so a small change in the <linear growth factor> causes a large fractional change. This statement compares fixed-mass bins in the same cosmology and concerns relative growth, not the time for one halo to assemble all its mass.
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