When a black peppercorn is 4.25 mm pinhead or larger, it's "Tellicherry." That's all there is to it. Because Tellicherry are so much bigger than the other peppercorns, they make up a much smaller percentage of the crop. Oftentimes they represent 10% or less of any given harvest. There's less of them, so command a higher price at market. More @Wikipedia
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