  We report the efficient production of a large Bose-Einstein condensate in
$^{87}$Rb atoms. This is achieved by quickly reducing the radio-frequency of
the magnetic field at a rate of -96.8 kHz/s during the final stage of
evaporative cooling, and we have produced a condensate with ${(2.2 \pm 0.1)
\times 10^{6}}$ atoms against a serious three-body recombination loss. We
observed the dependence of the cooling efficiency on the rate at which the
truncated energy changes by measuring the condensate growth for three kinds of
radio-frequency sweep. The experimental results quantitatively agree with
calculations based on the quantum kinetic theory of a Bose gas.
