Having greater Toa power, e.g. through the Nui Stone, allowed a Toa greater control and utilization of the elements. A Toa whose Toa power was reduced or used up in some way would be weaker in general.
Toa could only put their Toa Energy into Toa stones. They could not put their Toa Energy into armor, masks, or crystals.
Toa could not sacrifice their Toa power into Kanohi.
A Toa who gave up half of their Toa power would be weakened.