Oh! Oh! had an epiphany while taking a shower. SHOWA POWA!

That like food "souls" give a different amount of energy by whom they're taken from. For instance a person that eats healthy, and is athletic will give Grimm far more energy then the "soul" of a over weight, drug addict. Grimm has a fast metabolism due to all the things that absorbs the energy he processes such as his suit, and scythe. The organ holds the souls temporarily as it processes the "souls" before it passes on into the rest of his body. In lamens terms it's like the human digestive system only the food is "souls"

I'm calling the energy he absorbs from his victims souls, because it is easier.
 
Before you ask again the max capacity to the souls he can have in this "soul stomach" is twenty souls, bad or good they are all the same size. Though the input of energy is less, and really bad souls can in fact do harm to Grimm's body. Though its like eating bad bread leaves him queasy, but otherwise perfectly fine. Though he is of course smart enough to know what souls not to eat.

Also sorry for the double post.