However, for such a thing to be possible, one would have to set the predetermined standard for the souls in advance.

Like a full glass of water.... No matter how much power you pour in, one cannot demonstrate one's power when that power exceeds one's capacity.

The overflowing power will slowly hurt the vessel of the soul, little by little.... and result in the destruction of both the soul and the vessel.

What is beginning to happen to Miss Marisa will eventually happen to all magic users. Killed by their own power.