Hydrothermal synthesis has been extensively utilized for fabricating carbon quantum dots (CQDs). Generally, the average sizes of the CQDs are controlled by using specific precursor concentrations, processing temperatures, and reaction times. In our study, the average size of CQDs can simply be controlled by using a different filling volume of sucrose solution in the hydrothermal reactor while keeping the other experimental parameters constant. If homogeneous nucleation plays a major role in the hydrothermal synthesis, the CQDs synthesized by using different filling volumes should have relatively the same size. Nonetheless, we found that the average size of CQDs is inversely correlated with the filling volumes. Particularly, for the hydrothermal syntheses with the filling volumes of 20%, 50%, and 80%, the average size of the CQDs is 15, 13, and 4 nm, respectively. Therefore, the hydrothermal synthesis of CQDs with size-tunability can be achieved by the heterogeneous process associated with the total surface areas between the precursor and reactor.