exodus 12:4b, "You are to determine the amount of lamb needed in accordance with what each person will eat."

This was during the passover and the amount of blood that was afforded each house was kind of directly based on how much the household could eat.  

Today, we're given communion in a fixed amount, by how much we eat.  It's kind of an odd connection between what we eat and the amount of sanctifiying blood we receive.  Kind of like a serving size.