None of these explanations show that it is better, cheaper, or even as good as just run your wires from larger to smaller. The only thing I am hearing is "Some guy speced it. Must be better".
The OP had seen this and the field and asked about it. I merely said that it's common on large jobs and have been told that it's a cost reduction measure. Is it better, cheaper or just as good as another method? That would be a good question for an engineering forum
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If you just sized the run for load and VD consistently for the whole circuit... Would it really cost THAT much more? That's a real question, not a rhetorical one. Seems like by the time you do all the complex calculations necessary to change wire size mid circuit and know what VD is going to be, buy 5-6 rolls of different sized wire, change the spools mid stream, supervise all this to make sure wrong size isn't pulled from point c to point d etc etc. would have been easier (and maybe cheaper?) to just say go grab that #4 and wire those lights. But then I suppose I'm no engineer, and I'm not wiring miles of lights.