It is common for patients to have dilation of the neo-aorta (former pulmonary valve and artery) after an arterial switch operation for d-TGA. It is very variable in degree. The z-scores in your child's case are large, and what your doctor will be watching for is: 1) the rate of change over time, 2) progression of aortic valve leakage. if the neo-aortic valve starts to leak alot then it may be time to operate to reduce the size of the aorta and support the valve. Fortunately it is very uncommon to need to do that. However, when you have a child with congenital heart disease of a significant nature s/p heart surgery, you will never stop worrying.