jmanque is correct. Balls don't swell when tied. They may appear to swell, as tying them off tends to stretch the skin. They'll also turn red then purple due to restricted blood flow.

Scrotal infusion doesn't cause balls to swell, either. Just the ball sac will get larger and that is through inflation created by the saline injection.

What does cause balls to swell is injury. A good way to cause that is percussion play. Tie the balls so that they'll stay in one place or hold them in a loop created by your thumb and forefinger. Take a light wooden or leather slapper, something the size and heft of a paint stirrer and start tapping them, increasing the swing over time and listening for a fairly constant "keening" sound from your bottom (sub or slave). Take things slowly and in a while you will be administering some pretty sharp whacks to the balls. You can move from your paint-stirrer to heavier thud with care. Your bottom won't particularly like this kind of play and I recommend a certain amount of restraint to keep him from walking off.

Do this twice daily for a week and you will get measurable swelling. I speak here from actual experience, having measured mine before and after one and two weeks of this kind of regularly-administered treatment.

Careful topping in this way will result in consistent sensitivity without serious injury. This did not lower my sperm count appreciably.