I think it grows, (dialates) because the wall of the aorta becomes stretched and compromised perhaps because of factors that are different for each person. An illness that affects the heart specifically, likeRheumatic fever(from severe “scarlet fever“), Marfans Syndrome( inherited disorder), or some kind of weakness in collagen.. these are the things that add to the blood getting in between the layers of the wall. The blood causes a bulge which can be measured. From what info I have gathered, at a certain “dilation” there is less blood flow to be distributed to the major organs. There can be coughing because there’s pressure on the esophagus. There can be shortness of breath. A person can get by with these early stages, and not seem to experience great difficulty unless they are a very good athlete. But it is a silent condition, therefore, a potential deadly one, and must be monitored as we become aware of it. It is good in that sometimes a happenstance shows the condition, like you are getting tested for other things such as lower abdomen scan or chest X-ray .