The lightest, most compact u-lock that gets a "silver" rating is the OnGuard Bulldog Mini TC...it is the standard Mini with the bulky plastic cover on the cross-bar removed. There is lighter or more compact u-lock that has dual locking bolts.

The Kryptonite Evo Mini is the lightest "good" u-lock, but it has only one locking bolt, which is probably a little less secure than the OnGuard two bolt design.

Neither of those locks is tough enough to use in downtown Houston if the bike will be out of sight for more than three or four minutes. They are "coffee shop" locks, that will keep your bike safe for a minute while you grab something to drink...if your bike will be parked for hours, you need the Fahgettaboudit or NY lock, or something equally serious.

Several of my bikes have rear racks and saddle bags. I've noticed that when a lock is carried on the rear rack, it doesn't matter what they weigh...you don't even notice they are on the bike.