  Validity of modified finite-size scaling above the upper critical dimension
is demonstrated for the quantum phase transition whose dynamical critical
exponent is $z=2$. We consider the $N$-component Bose-Hubbard model, which is
exactly solvable and exhibits mean-field type critical phenomena in the
large-$N$ limit. The modified finite-size scaling holds exactly in that limit.
However, the usual procedure, taking the large system-size limit with fixed
temperature, does not lead to the expected (and correct) mean-field critical
behavior due to the limited range of applicability of the finite-size scaling
form. By quantum Monte Carlo simulation, it is shown that the same holds in the
case of N=1.
