The size of your Dantians depend on how much effort you build trying to cultivate them and your energy.
Energy moves through the body from the lower to the middle to the upper Dantian. When we practice, we build energy in the lower Dantian. Any surplus in the lower Dantian expands up to the middle Dantian to support our health, organs, blood, and respiration. Any surplus once our good health is achieved positively impacts your thinking and emotions. Now the surplus moves to the upper Dantian in order to replenish what is missing, often the ability to concentrate or focus. Basically, the energy keeps you from being overun by emotions and thoughts, and rids you of “monkey mind”.