Can a simple/complex cyst in a kidney ever shrink?

Possible. But unusual. If the complex cyst was due to blood in cyst, over time the blood could resorb and the cyst shrink some. If a simple cyst ruptures spontaneously which is rare, it could disappear. A complex cyst needs a follow up however to rule out risk of malignancy.
See below. Simple cyst may shrink, and in some cases get bigger. A complex cyst needs to be followed for growth and any signs of malignancy.