As the wall of the blood vessel weakens, that part of the artery expands.  Over time as the artery wall expands it becomes thinner (imagine the walls of a balloon as it gets increasingly inflated). The abdominal aorta is approximately 1 inch (2.5 cm) in diameter; aneurysms can expand that diameter to 2 inches or greater (5 cm or more) at which point they have a much higher risk of rupturing.