Well, I'm guessing a little about how much size/weight can be saved by designing lenses for smaller sensors, but intuitively we can all see that lenses generally can be smaller and smaller as the sensor size gets smaller. The lenses for my micro-4/3 system are a lot smaller/lighter than my Nikon DX system, and Nikon 1 lenses are even smaller, etc. The area of an APS-C sensor is
57% smaller than a full frame sensor (
full frame 864 mm2 vs. APS-C 370 mm2) but the potential reduction in the weight would not be
that much. The focal length after all remains the same, but the size of the lens elements and lens housing can get a little smaller in order to deliver the image to a smaller sensor. --Dave
I'm not an expert in optics, but as I understand it, with long lenses, particularly fast-aperture ones, the size of the front element and the rest of the optics become the limiting factors, and it becomes difficult to realize any benefit from reducing the size of the image circle. For example, a 300mm f/4 lens will require a front element of at least 75mm to achieve that aperture (300/4), hence the existing 300mm f/4 AF-S is already around the minimum possible diameter, since its front element takes 77mm filters. Likewise, none of the telephoto lenses on other systems that are specifically designed for crop sensors are much smaller than their full-frame counterparts. For example, the Olympus 300mm f/2.8, designed to cover the 4/3 sensor lens is actually physically larger and heavier than the full-frame Nikon 300mm f/2.8 AF-S VR II (3300g vs. 2900g).
Instead, the only way to shave weight and bulk off of a large telephoto lens is to either a) use lighter structural materials and revise the optical design such as in the latest Canon 500mm f/4 you mentioned, or b) use various optical tricks such as including diffractive optic elements, e.g., the Canon 400mm f/4 DO IS, which is much lighter and shorter than the closest conventional equivalent (300mm f/2.8 + 1.4x TC), although the front element is necessarily still 128mm wide. But either of these methods can be applied regardless of whether a telephoto lens covers a full-frame or crop-frame sensor.
On the telephoto end, the only size benefit that DX confers is the ability to take advantage of the crop factor to get more pixels on a target for a given focal length. When I replaced my D7000, I almost went with a D800E because it provided the same pixel density as the D7000, but ultimately went like you did with the D7100 because it provided 24MP in the APS-C frame versus the 15MP of the D800E.
Conversely, for shorter lenses, the front element diameter ceases to be a limiting factor in size (e.g., even an 85mm f/1.8 only needs at least 48mm), and particularly for mirrorless designs, the shorter flange distance makes it easier to design wide-angle lenses. And it definitely is possible to make high-quality smaller DX lenses covering the wide-to-normal ranges; I also have the Tokina 11-16 and Sigma 18-35, and agree that they are excellent. Aside from the 35mm f/1.8 and arguably the 10-24 and 16-85, Nikon has just completely failed to do so; I've always used Sigma, Tokina, or Tamron glass for my shorter DX lenses.
Also, like you, for a long time I wanted a high-quality wide-angle DX lens. If it had come out a couple of years earlier, I absolutely would've picked up the Samyang 16mm f/2 the moment it came out. But today, I find that the answer for a fast wide-angle need is addressed either by bringing along the Sigma 18-35, or more typically, by also bringing along my Micro 4/3 kit and the 12mm f/2 to cover all focal lengths except for telephoto.
