The other issue with using lower capacity packs comes into play when braking. When you hit the brakes in a brushless vehicle voltage and current is created by the motor, just like the regenerative braking you hear about with full size electric and hybrid cars. In fact most esc send this power back to the battery as the esc simply cannot absorb the power created and it has to go somewhere. Now if the battery cannot take the current quickly (NiMh or a low capacity/low c rated lipo) the esc will be stuck with absorbing it. If the esc cannot absorb it then obviously there's going to be a problem.