People with AF often also have narrowing of the valve between the left atrium and the left ventricle. This causes the wall of the left atrium to become thicker. Suggest why.

With a narrowed (stenosed) AV valve it is more difficult (requires more pressure) for the blood to pass from atrium to ventricle. The blood pressure in the atria is increased.

This means that the atrium must contract harder and with more force in order to pump the blood through the narrowed valve into the ventricles.

And becuase the muscle wall of the atria is working harder it begomes thicker.