What does it mean/ what causes a cyst to get bigger?

Several things. Cysts are fluid containing lesions with a thin lining of cells. Cysts can occur almost anywehre in ovaries, breasts, bones, liver, spleen, thyroid gland, kidneys neck, etc. Usually cyst grows slowly over time with fluid usually secreted by cyst lining. Sometimes more rapid growth can occur with superimposed infection or hemorrhage from trauma. Most cysts are benign.