Hello and welcome to e health forum.
During the developmental stages, of the fetal embryo, the organ grow and change in shape as well as position. Hence it is common that the position and shape and size of organs can vary in every individuals. In addition, No 2 kidneys in the same person can be identical.
With regard to your question, since your renal function tests are normal, and the ultrasound has shown no other defect other than small size, there is no need for any worry or concern.
Unless there is any symptom or sign related to renal condition or change in parameters od renal function ( Urinalysis, Creatinine and BUN), there is no need to do any further studies or tests.