My Dozer had a cyst pop up and it slowly, over a period of 4-6 months, grew to the size of a small marble. We had it needle aspirated and it was full of what looked like dirty serum. The Vet drew off a lot of crud to the point that the cyst was about 1/4th its original size. Much to our surprise, within 3 weeks it had grown to the size of a ping pong ball. We had it removed along with another smaller cyst.
It's entirely possible that the cyst was caused by his harness. However, he was wearing only a Martingale Collar when we first noticed it(he had recently switched).....the new wife did not care for harnesses...and she knows best.