I've weighed most of my locks at the Post Office. The heaviest and largest Kryptonite and OnGuard u-locks came in between five pounds and six pounds. The very lightest u-locks from Kryptonite and OnGuard came in at about two pounds.

When the lock is sitting on a rear rack, the difference between six pounds and two pounds is not a big deal. But, on bikes without a rack, I hang the u-lock on the bars next to the stem...and a six pound lock is VERY noticeable when turning a corner.