Okay.
If you were to take a 10lb weight that was 1ft in diameter and put it on your arm, the center of mass of the plate would be 6" in, and since that would be 6" towards your elbow you would have a small lever, and it would "feel" lighter than if you held a 10lb plate in the palm of your hand.
When you turn the weights sideways to put them on a bar, relize that the center of the mass is still the center of the plate, but all the force is transferred to the bar. Thus, regardless of the diameter of the plates, the bar is the mechanism through which you exert force. Since that is a constant, the diameter of the plates doesn't matter. See what I mean?
With your 7ft bar example, if you pick up one end of it, it IS a lever -a very long one at that. Any weight on the end will have a tremendous moment arm from which you would have quite a lot of force to resist. When you grab it in the center it is balanced, no moment arm. The "distance" would only make a difference if the opposite side wasn't balanced.