You eat food, and you don't save any room, you eat and eat until you are full and satisfied.
This is when the problem begins because, your stomach still needs to make room for water to hydrate you, and also room for the expansion of gases so you can digest the food. Basically meaning now that you have eliminated the room for digestion and water, your stomach has no choice but to expand.
After a large meal, the stomach muscles expand up to 3 times the normal size pressing against the lungs. This is what causes a person to feel out of breath after eating a lot.
When it’s hard to breathe, your taking in less oxygen which means your brain receives less oxygen making you feel tired. The body also has to spend more energy to digest the food leading to an energy drain and adding to that sleepy feeling you experience.
Giving your body a constant supply of energy also means it won’t have to dip into your fat stores for energy. If you constantly give the body more energy, meaning food, than it can use, it continues storing extra energy increasing your fat stores. By constantly eating, your body continues to store fat that it will never use.
The more you eat until you are stuffed, the more you force it to stretch out, the more it stretches the more appetite and the more food you can put down before you stuffed again. Then you feel too sleepy to work it off and take a nap only to wake up hungry and start all over. It becomes a vicious cycle where you can eat more and more and you gain more and more weight.
The brain helps this cycle to continue as well. Dopamine, a chemical which is found in the human brain, induces a feeling of happiness when you eat. However, when you overeat the dopamine levels in the brain get depleted. When the dopamine levels get depleted, you have to eat more food to get a feeling of satisfaction.