[BktBallRef]

I really don't understand what you mean by "leaning over of the defender." There's no rule that forbids a taller opponent from reaching over or leaning over a shorter opponent. Unless he displaces the opponent or limits his movement with contact, there can't be a foul.

"a defender cannot obstruct an offensive players motion unless they have given enough space for the offensive player to alter their motion."

I think this is what you're talking about.

A defender can obtain any spot he desires on the floor, as long as he attains the spot legally. If you're gaurding a player with the ball, no time and/or distance is required. If guarding a defender without the ball, some time and distance is required.

For example, if you're driving with the ball and I step in front of you, face you, and have both feet on the floor, and you plow into to me, that's a charging foul on you. There is no time and distance required.

If you don't have the ball, then I must give you time and distance to avoid making contact with me. But the distance need not be more than two strides, depending on how fast you're moving. But once I have my spot, I can move to maintain it.