Good answer!
But to help out the Young Spat on this... it is harder to produce larger pearls: thus they are less abundant and therefore more valuable.
In the "old days" (up to the 1980's) most South Seas Pearl farmers would have a good supply of wild-caught pearl oysters, many being rather large because of their age (up to 20 cm in diameter). Their larger bodies being able of accommodating a larger nucleus thus producing a larger pearl.
Nowadays, most pearl producers employ lab-reared "spat" (oyster juveniles) and rear them in their farms, and some years later (depending on the species, the site and other factors) the oysters have grown enough to be "seeded" to grow pearls...but they are rarely as big as the wild-caught oysters (10-14 cm), so the first pearls they produce are simply not as big as those of the former... you need more time.
And -as the saying goes- "Time is Money".