My kidney is smaller than normal what does it mean?

You are unique: If your blood tests (bun/creatine) are normal, then it is doing its job adequately, and nothing to worry about. If you mean that one is substantially smaller (like 1/5-1/10th) than other side, it is more likely from a developmental issue before you were born, and the same applies for blood tests, since we can lose/donate one kidney and function normally off the other.