I was looking at long zoom lenses for Canon in the last week at saw that lenses that go from 18-135mm weigh about the same as for 18-200mm, 18-250mm, 18-270mm. Of course the longer the zoom, the more expensive, but I'm wondering why they weigh the same (400-450grams). I'd assume that the longer the zoom, the more glass you'd need. The more glass you need, the heavier it should be. Thus 18-55 < 18-135 < 18-200 < 18-250 < 18-270, right? But it's not so. In fact, the 18-55 kit lenses for all brands comes in at about 200-230 grams, which is only half that of Tamron 18-270mm and the Tamron also includes ultrasonic auto focus motor and stabilisation.
Any ideas why they weigh the same?