That does sounds like an unusual amount of growth. I believe the recommendation is for surgery when the growth is greater than .5cm in a year. (https://www.ahajournals.org/doi/epub/10.1161/CIR.0000000000001106) My understanding is that CT is more accurate than ECHO. There is a lot to know about measuring aortic aneurysms. I think only a professional can interpret the readings. FWIW, I don't think the readings are ever accurate to a tenth of a centimeter, I'm just not sure how far off they can be. (I have tried to find something that pins it down, but haven't been successful. I believe it is somewhere around .2 or .3 cm. ) There is also variation according to who performs the measurement. I gather they often try to have the same facility/person do the measurement each time.

I assume you went to Cleveland Clinic because of their reputation. If it were me, I think I'd just have to trust them to some degree and ask my physician there about the very unusual amount of growth in a way that makes me comfortable that they are aware of how different their readings are from the ones that were performed previously. Almost surely their process is optimized for accuracy and consistency (though no one is fallible). Did you send them your previous measurements? 5.3 is notable, did anyone get back in touch with you?

I got an unexpected spurt of growth though not anywhere as large as your measurements indicate. I find it hard to have so little understanding of, and control over, a process that is so important to your life (literally).