What causes ganglion cysts to get bigger?

Movement: Ganglion cyst has been thought to derive from from tendon sheaths and joints due to there close proximity. Therefore it was noted that the size of the cyst may vary with time and increase after activity. I hope that this answered your question.

Synovial fluid: Fills ganglion cysts. These cysts are fluid filled sacs and synovial fluid is what lubricates the joint, fluid enters the cyst through a tiny attachment from the joint and makes the cyst large . This is why activity or force seemingly makes the ganglion larger as more pressure in the joint leaks into the cyst. Typically it's a like one way valve into the cyst but they can drain back spontaneously.