Well, what I can tell is that few men had a very nice moving balls that can give me a great sensation that is great when it happens.. but at least in my understanding it much depends the arousal stage he is in, because by the time he is really aroused his balls are tighter in him and then more compact as not doing the great slap.
I noticed that a guy with balls not hanging that much was the one who gave me the unforgatable slappings just because we started sex with doggy..hmmm..than if doggy was left for later after other positions etc I would not have felt his balls the way I did ? Dont know..as i never had a huge ball guys experience but bit low hanging kind of empty balls that became just different aroused, yep.
Maybe..and I say maybe balls behaviours are just like penis as the growers not showers ones, so interesting you can not tell his true size when it is sleeping lazy, after all...as we can not assume nothing much until we see it in action.
I wish to observe a fat big balls that hang lower and kind of not change much during the whole action, well sure just before ejaculating would be concentrated and tight, of course...but still a steady slaper balls, in her own free rights!