  We numerically investigate quantum quenches of a nonintegrable hard-core
Bose-Hubbard model to test the accuracy of the microcanonical ensemble in small
isolated quantum systems. We show that, in a certain range of system size, the
accuracy increases with the dimension of the Hilbert space $D$ as $1/D$. We
ascribe this rapid improvement to the absence of correlations between many-body
energy eigenstates as well as to the eigenstate thermalization. Outside of that
range, the accuracy is found to scale as $1/\sqrt{D}$ and improves
algebraically with the system size.
