My optical cost principle
I have mentioned this before, but it bears repeating:
The Optical Cost Principle:
The cost and weight of optics goes as the cube of the linear dimension of the sensor (or film format).
There is probably already technical term for it. If there isn’t, you can call it ”Terry’s optical cost principle.” 
[Discussion after the jump]
Discussion
The principle is common sense. Basically the cost and weight (which is what you care about) goes as the size of the optics which goes as volume which is dimensionally length3. The only significant linear dimension is the sensor size (delta some small factor differences depending on if you are wide aspect or not).
What this means is that as your sensor gets bigger, the optics to support it gets more expensive, heavier and less practical.
The only exception to this is when comparing lenses that are stop gaps for lenses that exist in the 35mm world, For instance the 12-24mm f/4G which replaces the 18-35mm f/3.5-4.5D at wide-angle or the 18-55mm f/2.8G which replaces the 28-70mm f/2.8D as a ”wedding lens”. These are unusually wide angles designed for a ”stop gap” in which the principle breaks down because of the need to have retrofocal optics to bend the light at an extreme angle, clear the mirror, and then collimate it for the digital sensor.
Instead, look at the 18-200mm f/3.5-5.6G VR which has no Nikon equivalent in the 35mm world, but if we take Canon’s 28-300mm f/3.5-5.6L IS as an example we can see reasonably, that the former costs $750 and weighs .6kg, and the latter costs $2500 and weighs 1.6kg, thus proving the optical cost principle.
When talking about APS-C, size is an important factor as any.
May 15th, 2006 at 3:19 pm
[...] Why not 35mm (8/3″)? I think Terry’s optical cost principal explains this very well. Translation: 6/3″ (APS-C) is much cheaper and lighter than 8/3″ (35mm), with very little quality loss. For content creators who need to go cheap and mobile, this this key. [...]
August 19th, 2006 at 9:50 am
[...] But Reichmann shows what a fucktard he is when he calls the size advantage “a myth.” He feels he blows a hole in my optical cost principle by comparing the Canon EF 24-105mm f/4L IS USM (on a 35mm body) to the Leica D Vario-Elmarit 14-50mm f/2.8-3.5 (on the L1), which even he admits is “a fair bit lighter.” See anything wrong here kids? [...]
March 9th, 2008 at 10:51 pm
[...] already explained the advantages of a travel kit lens before. Basically, if you want this lens for “full frame” you are talking about an optic that cost and [...]