Chopped glass construction is weaker because instead of the fiberglass strands being long and woven they are short and bound together only by the resin matrix. Chopped glass construction usually also results in a resin/fiber ratio that is much higher than ideal. Too much resin makes the canoe heavier and more brittle.
The weight of an all-fabric fiberglass or composite lay-up will depend on many factors other than the size and shape of the boat. Even if we consider only an all-cloth fiberglass boat, the variety of fiberglass will be one issue. S-fiberglass is about 20% lighter and stronger than the common E-'glass. Of course, the weight (ounces/square yard) of the cloth will matter, as will the number of layers and whether these layers are full blankets or partials. The use of fabric allows more thickness to be positioned where it is needed and less where it is not.
The construction technique also matters. With hand lay-ups the cloth is placed in a mold and resin is applied by hand using rollers and squeeges or the like. The resin/fiber ratio can be improved further using more advanced techniques such as vacuum bagging and infusion technology. If you are interested in those methods you can search google.
If you want to reduce weight further, fiberglass can be combined with other materials such as aramids (e.g, Kevlar), other synthetic fiber fabrics such as Innegra, carbon fiber, polyester, and other materials.
Ultralight canoes typically use some type of a foam skeleton of a bottom core and side ribs to provide stiffness while allowing the number of fabric layers to be reduced.
Trim selection (gunwales, thwarts, seats, etc) also comes into play. Most composite canoes are gel-coated which provides for a nice, glossy appearance. But the gel-coat adds weight but little or no strength. Omitting the gel coat in a skin-coated boat will save several pounds or more.
Here are a couple of examples of 15 foot composite tandems. First the Nova Craft Bob Special which is available in 5 different lay-ups:
You can read about the different lay-ups here:
The all-fiberglass lay-up is the heaviest and least expensive and weighs 58 lbs, which I would say is fairly typical for a 15’ tandem canoe.
Here is a Colden Starfire, which is a gel-coated, all fabric, fiberglass/aramid composite hull built using resin infusion technology:
The boat weighs 45 lbs with wooden gunwales. The weight can be reduced further with composite rails.