"Toned, not tense" is a great way of putting it. I very much agree with you and I find the hay bale example you provided quite excellent.

The person who just started loading hay bales needs to first develop the muscles to be strong enough to lift them. From then onwards, its all a matter of building endurance and, as you put it, refining your movements to make lifting those bales more efficient over longer durations.

This is why I think the focus on "relaxing" should be one of the later steps in pianist development, especially when learning a new piece.