The size of a voxel (usually given in cubic mm) defines the spatial resolution of an imaging experiment. Let's assume that the signal strength in a single (isotropic, i.e. all edges have the same length) voxel is proportional to the number of molecules in it. A high spatial resolution is preferable, so one wants to reduce the voxel size.
How does the signal change if the edge of the voxel is reduced by 20%/50%/80%?
Hint: Reduction in signal = [1 - (1 - reduction%)^3]
If you solve this equation, the answers are 0.488, 0.875, and 0.992 for 20%, 50%, and 80%. However, if the voxel is reduced, shouldn't the signal increase?