Balenciaga bags are lightweight, and I thought they were the lightest (in connection w/its size) until I got a Gustto Setela which is a large bag that is extremely lightweight. The leather is soft and thin and just molds into my body. Even with the bag pretty loaded with "stuff", it's still the lightest bag I've ever carried (including my Balenciaga City).
Gustto's Baca bag is also lightweight and can hold a lot for a "small" bag.