OK, so 9 inches is not that giant, but the California Abalone - Haliotis Rufescens is substantially bigger than Abalone in other places around the world. A recent article suggests that this is due to sea otters.
"We realized with this study that most abalones are small, averaging 2 inches across, and live on coral reefs in the tropical Indopacific," Lindberg said. "So why does the world's largest abalone live in the same place as this major predator, the sea otter? We think the abundance of kelp, the ability of abalones to stay hidden in crevices, and the predation of otters set up an ideal system to ratchet up the size of abalones."
I'm not sure I completely agree with their speculation, but the history of the interaction between kelp, sea otters, abalone and sharks is interesting reading.
My picture above shows the relative size of abalone with the shell of the abalone from the Northern California coast completely enveloping some abalone (awabi) shells picked up on the beach at Hayama on the East Coast of Japan. The larger shell is about 9 inches across and was harvested near Fort Ross. The little ones are one and a half to two inches. They are much more delicate as well. Of course if you only care about eating I direct you to this recent blog entry by one of the better food bloggers http://epicureandebauchery.blogspot.com/2006/03/awesome-awabi.html
