Atheroma not only narrows the smaller arteries (such as the coronary arteries around the heart), it also weakens the walls. This means, that in larger arteries, there is no significant narrowing, but the wall is compromised. The biggest artery of them all is the AORTA. It carries all the blood that comes out of the heart, distributing it to the arms, the internal organs, and then to the legs; this last it does by splitting into 2 ILIAC arteries.
The weakening of the wall caused by atheroma and the pressure of the blood as it is pumped from the heart causes the aorta to expand. It expands either as a balloon to one side, or as generalised expansion of a segment. This expansion is the aneurysm. It usually affects the lower aorta and one or more iliac arteries, but may occur anywhere along the aorta, including the bit in the chest. Unfortunately, the laws of physics mean that the larger the aneurysm gets the easier it is to expand (think of blowing up a balloon).
When the aneurysm bursts, there is a catastrophic loss of blood, and is a frequent cause of death.
Atheroma is not the only cause of weakness in blood vessel walls, but it is the commonest, especially among the elderly. There is a screening programme for aortic aneurysm, and it can be treated surgically before rupture; this operation has a much lower mortality than operating on an aneurysm that has already burst.