The National Kidney Federation explains that the most common reasons one kidney is larger than the other include congenital dysplasia, reflux nephropathy, kidney infection and blood starvation in the smaller kidney. Diseases such as glomerulonephritis also cause a kidney to become smaller.
Congenital dysplasia is a condition wherein a kidney is small at birth or has never grown with the rest of the body, says the NKF. It is a common cause of small kidneys and is typically detected in childhood. In some cases, the small kidney is in a normal position in the upper back. However, in other cases, it is found in the lower abdomen as it has failed to move up from there before birth.
Reflux nephropathy refers to a damaged kidney due to a faulty drainage system, explains the NKF. It is commonly detected in childhood and early adulthood.
According to the NKF, sometimes one kidney is bigger than the other because an infection has caused the other kidney to shrink. Severe kidney infections damage the kidney and cause it to become small. When the kidney lacks blood due to a narrowing in the artery that supplies the organ with blood, it often becomes smaller. This condition typically occurs in older people, especially those with a history of angina or heart attack.