I dont think this it is fair to assume, that 39mm is optimal for people because it's used more; might instead be optimal for the purchasers/casinos because its less clay and easier to store.
This is something I’ve actually researched for my long-running book project... Amazingly, the same 39mm/1.5" size has been the standard going back almost 150 years—no matter what material is used, including early “clays,” acetates, various composites, bakelite and other resins, ceramics, plastics/injection molds, etc.
If anything, the cost and availability of materials has steadily gone down over time, making economics less of a consideration. My research suggests that the widespread, parallel and virtually unanimous decision to settle on this 39mm/1.5" size arose first and foremost from ergonomic, not economic, considerations which developed organically over decades of trial-and-error.
And if economic and storage considerations were the main factor, you would expect companies to be using the major advances in both material options and mass-production techniques to be shaving a mm or two here or there off the 39mm standard—same way that cereal boxes keep getting verrrry slightly smaller with less and less actual cereal inside the bag.
You’d also expect them also to try to make chips *thinner*, if even just by a fraction of a millimeter... But neither is happening. Why? I believe it’s because the standard dimensions and thicknesses used by Paulson/GPI, ASM/
CPC, etc. are not just familiar to people, but actually ideal from a usability standpoint.
Both smaller and larger size chips that were tried at various times have by now either been eliminated completely, or else relegated to specialty runs (e.g. chips for children, the occasional 36mm tourney chips, and more gaudy showcase chips at 43mm+ sizes).
Much the same can be said about cards: The standard size and ratio of dimensions (what we now call “poker size”) has been shockingly almost completely unchanged since the late 18th century. It’s hard to think of many other products which have been so consistent overtime.
Card makers, too, had every economic incentive to make cards smaller, especially back when cards were made of laboriously-produced papers with a high cloth content, and strenuously inked by hand or on old-fashioned presses. Yet once the current standard size was arrived at (through another long process of trial-and-error), there was a widespread recognition across manufacturers and regions of the world that that this was the best size for the most people.
It’s a lot like countless other everyday objects such as doorknobs or computer keyboard keys or mop handles... After decades or even centuries of making such things, producers of these goods see what the optimal size range should be for the largest percentage of users. You can deviate from them, and sometimes manufacturers do; but people are not just going to dislike the change from what’s familiar, many are actually going to have trouble using the product.
