What causes kidney cysts and what symptoms indicate them? As people age, kidney cysts can form on the surface or in the nephrons of the kidney. They can vary in size from a small pea to a large grapefruit. Kidney cysts are usually small, round sacs that are filled with watery fluid. Most of those who have them don't know they have them. If the cyst ruptures and bleeds or grows large enough to press on other organs, treatment will be required. Kidney cysts are usually small, r