People tend to buy the big clunky U-locks because they don't know how to use them properly. A U-lock should go around the rear rim and tire, somewhere inside the rear triangle of the frame. There is no need to loop it around the seat tube as well, because the wheel cannot be pulled through the rear triangle.
Some will object that felons might cut the rear rim and tire to remove the lock. Believe me, this just doesn't happen in the real world. First, this would be a lot of work to steal a frame without a useable rear wheel, the most expensive part of a bike, after the frame. Second, cutting the rear rim is much harder than you might think. Since the rim is under substantial compression due to the tension on the spokes, it would pinch a hacksaw blade tight as soon as it cut partway through. Then there are the wire beads of the tire, also difficult to cut.