Akoya pearls are one of the smallest of all cultured pearl types, typically ranging from 5.0-5.5mm up through 9.0-9.5mm and very rarely in the 9.5-10.0mm - or even more rarely 10.0-10.5mm. So if all other grading factors are equal - luster, surface quality, matching, etc. - then the larger pearls are going to be the more expensive ones.
Additionally, you will start to notice that after the 7.0-7.5mm size has been reached, prices start to increase exponentially - similar to how diamonds are priced. So an 8.0-8.5mm Akoya pearl necklace will nearly double in price compared to the 7.0-7.5mm version, all other factors being equal.
This is because the larger pearl sizes 7.5-8.0mm and up require much longer time in the water to grow. This increases the risks of storms, red tide blooms and other unexpected calamities, potentially wiping out entire harvests - and the farmer bears the entire cost.
Additionally, the longer growth times require more care and resources invested in the oysters’ health, with daily checks, regular cleaning and investments in state of the art technologies to ensure that the farmer will maintain a nice, healthy crop of pearls to harvest.
At the very largest and rarest sizes of 9.5-10.0mm and 10.0-10.5mm, the size of the oyster itself becomes an issue - as the smallest of all pearl-bearing mollusks, most Akoya pearl oysters are simply unable to accommodate a mother-of-pearl bead nucleus of that size, drastically reducing the availability of oysters able to be used to culture these rare pearl sizes, which will command premium prices for fine quality pearls.
We will always recommend deciding upon your budget first, and then choosing the highest quality Akoya pearl jewelry you can buy within that range - even if it means stepping down in size. A highly lustrous, glowing strand of smooth, unblemished pearls will not only be more beautiful than a large but dull or pitted pearl necklace, but it will also be more beautiful and durable in the long run.