My understanding is that atrial enlargement is a response to high blood pressure. High blood pressure can be a systemic problem, maybe related to reduced kidney function (the kidneys are key in controlling BP), or it can be a problem with the left ventricle, or even slow/stiff valves that the ventricles are trying to force the needed volume of blood through. If the left atrium is what is enlarged, chances are that it is being 'blown back' or 'blown up' by the powerful left ventricle below it. The mitral valve between them is meant to be a one-way valve. The left atrium receives oxygenated blood from the lungs, and pumps it one way through the mitral valve into the left ventricle. That's the 'lub' in the 'lub-dub' of a heartbeat. The left ventricle receives its electrical stimulation a fraction of a second after the atrium does, and it contracts forcefully. Sometimes too forcefully and it may enlarge in time as a result. But let's deal with the enlarged atrium: the ventricle contracts, and like any fluid, the blood will flow into the path of least resistance. In a normal heart, mitral valve in good order, the blood surges up into the aorta, curves as the aorta curves, and branches off blood to the various arteries coming off the aorta. When the mitral valve is compromised, it leaks. It doesn't keep the surge of pressurized ventricular blood from being flushed back into the atrium. The atrium is trying to refill with venous blood from the lungs (yes, 'venous' because all blood flowing to the heart is 'venous', meaning the pulmonary veins return oxygenated blood to the heart, which everyone needs to happen). But the ventricle is also forcing blood back into the atrium through the weakened/prolapsed mitral valve. If the left atrium is beating out of rhythm due to AF, it might beat at the same time that the ventricle beats. Guess who wins! The larger and more powerful ventricle inflates the left atrium as if it were a balloon! Eventually, it literally enlarges. But, in many cases, with corrected AF, the left atrium can reduce in size a significant degree. This is good!
A pacemaker is not a guarantee that he will be free of AF. What it might do is to reduce the rate AND the frequency quite a bit, which will help. It also can pace the left ventricle to not be out of sequence so often, or to not speed up with what is called 'rapid ventricular response' (you can google that phenomenon).
Lastly, please note that I am not an expert in this: I have no formal training.