Because we mostly breathe with our [diaphragm](https://maidstoneosteopaths.com/wp-content/uploads/2020/05/Breathing-in-and-out.jpg), a muscle surface that sits below the lungs and above the stomach. So when the stomach is full the diaphragm can’t move down as easily, no space.
You can also use your chest muscles / ribs to “inflate” your chest to breathe, but it’s more “effort”.
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