Could a kidney becomes smaller after diving?

No, not from diving: No reason for it. You would have to have had ultrasounds, x-rays or mris at least a few month apart to determine whether a kidney had become smaller. Most common reason for "apparent shrinkage" is 2 ultrasound studies performed by different machines, whereas there might have been no change if study repeated using same machine.Reasons for kidney shrinkage are pyelonephritis, diminished blood supply.

Increased : Atmospheric pressure from diving should not shrink kidneys.

No: No change in the size of solid organs is seen from diving