A few textbooks state that newborns tend to be large with transposition of the great vessels, but none explain why that is the case?

Diabetic mom: Infants of diabetic moms are at greater risk for congenital anomalies, including heart defects. Infants born to diabetic moms are also more likely to be larger.

Not directly: Transposition should have no real effect on the fetal circulation and should not affect fetal growth. However, large babies are more likely in the setting of problems with glucose metabolism such as gestational diabetes, which is an important risk factor for congenital heart disease --> higher risk of birth defects like tga in large babies. That's the best i can do...