Does it mean something if one kidney is slightly bigger than the other?

Asymmetrical kidneys: In general most kidneys are the same size. If the difference is more than a centimeter it may be significant. You may have been born that way. There may have been prior infection scar or decrease blood flow to one kidney. Also an ultrasound is technician dependent so maybe it's the test was done and not accurate.

See below: It may be part of development, or it may be one is shrunken due to damage from lack of blood supply, or one is blocked and is swelling up [called hydronephrosis] your doctor can help.