The size would be given by the diameter -- you'd say, "I'd like one 90cm in diameter". But the quantity of materials used -- the crystal components and the metal, in this case -- is determined not by the diameter but by the area of the surface of the sphere. If you double the diameter, you more than double the surface area of the sphere.
At school, you learnt that the surface area of a sphere is given by the formula 4πr². The radius is half the diameter, so doubling the diameter doubles the radius.
But there are those other factors in the formula. So, for example, if the radius is 50, the surface area is 31,429 (50 x 50 x 22 ÷ 7 x 4). But double the radius to 100, the surface area becomes 125,714 (100 x 100 x 22 ÷ 7 x 4), which is not double, but four times the original area.
2. The unit cost of the raw materials and components will be higher
It takes on average two years for a lighting maker to bring an original concept to market. Many processes are taking place during this time, that are concerned not just with its design, packaging and pricing, but also with what it will be made from, which subcontractors will be used, and how it will meet the relevant regulations (the main one of which -- IEC60598 -- is 192 pages long in its English version).
The prices of raw materials and sub-assemblies of the standard items are therefore tightly controlled. But a special undermines all this work.
The larger the stock order is of, say, crystals of a particular type, the cheaper they will be. But, if the maker has to buy in a small quantity of something they don't stock, the unit cost will be higher.
Some items -- for example, fabric covers for cables -- can only be bought in minimum lengths, which could be a kilometre. So, if only 1m is required for your special, they still have to buy 100m.
3. You can't expect people to work for nothing
Once a catalogue item is launched, pretty much everything has been worked out. Issues will come up, of course, but there will be standard computerized systems controlling the stock, subassemblies and manufacturing. Packaging is designed and sitting on shelves. The light meets all the regulatory requirements.
As soon as we ask for something different, however, a miniature version of all the work done for the catalogue item has to be done again. The implications of the modifications have to be identified and designed around. Greater weight may mean that the suspension components have to be changed and other modifications made so that the tilt tests are passed. Once quantities have been calculated, suppliers and subcontractors have to be negotiated with, specifically, and only, for the special.
The modified version has also to meet all the regulations.
Only highly skilled people can take care of all these things.
And they are not sitting around doing nothing, awaiting our email. They may have other work to finish before they can look at our special. So might the subcontractors.
Then, making the special has to be scheduled through production. Not only has a window to be found for it, but it will take longer than a standard item and may need to be done by the most senior operatives. They will have to wait until the special parts have been delivered and tested.
...no. It will cost more than twice as much. And you will have to wait for the experts to design and cost your special. It will also take longer to make.
Always specify a standard item if you want to keep the cost down and if you want a price quickly!