[CortezPearls]

Tahitian producers are depending more and more on hatchery produced spat, whereas in the 1980's and up to the mid-1990's they were still using a good amount of large wild caught black-lips. The wild oysters are much larger than farm raised ones (being older) so it was easier to produce larger pearls on larger animals.

Now thay have medium sized MOPS (mother-of-pearl shells), thus sizes are coming down. This combined with what Jeremy is saying about the second graft it just makes it easy to understand why larger sized blacks are "shrinking".