An echocardiogram report always comments on the size of the heart chambers. Your echocardiogram showed the top chambers of your heart are enlarged.
It is highly likely that your atrial fibrillation is the result of your enlarged atria not the other way around. Your atria have become stretched out. This is most likely due to higher-than-normal pressures within them. That has disrupted your electrical system in the upper chamber of your hearts. And that leads to the chaotic rhythm known as atrial fibrillation.
Why did your upper chambers become dilated? The higher-than-normal pressures might be from an abnormal valve between the left atrium and left ventricle (mitral valve). When this valve narrows or leaks, it leads to higher pressures and enlargement of the left atrium. Over time, those higher pressures can lead to higher pressures on the right side of the heart, too.
Theres another common and important cause of enlarge atria: thickening of your heart muscle in the lower chambers the ventricles. This is most commonly caused by high blood pressure. But disease of the heart muscle itself can result in stiff, thickened ventricular walls. This thickening causes the heart to have problems relaxing between beats. This leads to higher pressures that can dilate the atria and lead to atrial fibrillation.
I dont think you should be alarmed. Just make sure your blood pressure is under control. And if you are on blood thinners to prevent clots, be sure to have your blood tested regularly to make sure your dose of blood thinner is correct.