Synovial fluid fills. Ganglion cysts. These 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 so they go up and down.
Maybe. They may diminish in size on their own. Excision is not necessary. Aspiration of the cyst with a needle is a simple way to treat it.