The productivity of pearls at levels of sizes is distributed pretty much like a pyramid, and only a tiny fraction of pearls grow to be very large. Above a specified threshold, just a slightly larger size can raise its value exponentially. This threshold differs widely depending upon pearl types. For example, high quality freshwater pearls above 10.0mm are rare, as are Akoya pearls above 8.0mm, whereas for Tahitian and South Sea pearls only above 12.0mm do the prices for size start to rise exponentially.
A photograph may very well not display the actual pearl size accurately especially online, and it is important that the pearl size be clearly indicated for any product. A buyer should bear in mind what size and kind of pearls they are looking for when comparing prices among different retailers.