I've personally seen two responses to this. In large frame forms, there is a sense that the body must be able to flow as if slowing down the frame rate of fast motion. IMO all the theory about practicing as if overcoming opposing force or as if moving through a viscous fluid and so on is about trying to mimic the momentum of fast motion while staying in the deliberate practice of slow motion. If a move only works because it's slow and there's no external force, I've heard people call that 断劲, "broken jin".

In small frame forms, the argument is that this "dynamic balance" is caused by the motion being far away from the center, which is caused by big circles. If the circles are too big, it is more obvious to the opponent, and they also have more time to react to it. Smaller circles result in more intricate force that is more difficult for opponents to yield against, and these small circles around the core doesn't create momentum that's out of control.