The three different measurements may be from measuring different parts of the aorta or measuring the aorta at different angles -- or maybe the three different images were taken at different points in time. If the three measurements were sequential, meaning that maybe the 4.5 was first, and then the 5.1, and then the 5.5, then it would seem the aneurysm is growing. CT is pretty accurate, so if you got a 5.5 on CT, then someplace on your aorta measured 5.5, and that is usually an indication for surgery. If you are feeling a sense of urgency about it, then I would press the cardiologist and the surgeon for more explanation. I would want to know the reasoning behind waiting or proceeding, either way. Again, if you're wondering what is going on, ask the doctors, and don't let them off the hook until you get the answers that you need. Part of their job is to explain to you their impression and plan.
They need accurate measurements etc for the surgeon to plan his procedure. A cath is risky, everything is, even crossing the road, but it's about the level of risk. A cath is very low risk.