[James98]

I was using force as the pressure you feel by the rim of the cup on your chest. I assumed you were complaining about more pain because of the rim digging into your chest. The air pressure on your breast could remain the same while the force on the rim increases. The low air pressure on your breast is what causes the stretching and growth. So it is independent of the feeling of the rim force. Clear as mud?

I have not seen evidence that waiting to completely filling a cup to move up is better or worse than switching early. There are lots of opinions, no demonstrated facts.