Are you referring to the aorta artery? There may be genetic factors involved, or high blood pressure. The enlargement can be classified as an aneursym (>4.0 cm) and grow to size greater than 5.0 (depending on what segment of the aorta). The risk with an enlarged artery is that it can rupture as the artery wall is stretched and weakened.
Treatment is to control blood pressure and avoid any heavy lifting of weights, etc. If the size becomes an issue, there is an operation to resection the aorta. Sometimes there is a problem with the aorta valve (two cusps rather than three), and that may or may not be an issue.