I wonder if it could be because they are so small, that the dosage is relatively much larger. Their metabolism is much faster than ours: those larger dosages may also be needed to get any effect?
First you have to determine whether something works at all. Then you can try dosages. But my implied point was that mice may necessarily need relatively large dosages, because it doesn't work on them otherwise. This faster metabolism thing is nontrivial: for instance, in humans, metabolic rate is coupled to the perceived rate at which time passes.
