My last ct scan of my ascending aorta was 3.8x4.0 compared to last year's scan at 4.1x4.1, any explanation as why dilitation apparently is shrinking?

Probably Unchanged: I suppose stopping smoking and drinking and lowering BP can shrink the aorta somewhat. But the CT scan does not necessarily sample the exact same area each time. I view these numbers as unchanged, and within the error of the measurement. So good news.

NotReal-onlyApparent: linear, not true area measurements: from different points each time, within 2-D (not 3-D) slice presentations of CT https://goo.gl/FFAAx4 density data & at whatever grayscale window settings selected each time. As always, best examine both sets of CT image data (ideally original adjustable data; not just downcoded image data) side-by-side, adjusting grayscale window settings & examine yourself.

Stable: Taking into consideration operator/ scanner variability, the difference does not seem significant. Would call it stable with the true trend being given by previous values, if available.

It's not: It may not be shrinking. This is less than 1/2 cm. This could be due to measurement variability between technicians or the view obtained and point in the cardiac cycle when the aorta was measured. It's essentially unchanged.