The frequency of unilateral small kidneys has been found to be about one in 500 autopsies.
[link.springer.com]
[…] in patients with hypertension and a unilateral small kidney.
[annals.org]
Duplex collecting systems, minor rotational anomalies, distal ureteric stenosis, renal hypoplasia (small kidneys), unilateral renal agenesis (missing one kidney), unilateral
[thinkgenetic.com]