What's the deal with those big ass Thai amulet necklaces?

Glad you asked! Those are Jatukarm Ramathep amulets. Who is Jatukarm Ramathep? Well, according to this recent Editoral, entitled, "The facts behind the Jatukam Ramathep talisman nonsense" in The Nation:

As a result they have confused the two guardian deities, Khattugama and Ramadeva, and conflated and corrupted their names, producing Jatukam Ramathep, a single deity without a historical background.

History or no history, if you want to jump on this tailsman fad, you can get yours on EBAY.
You can even get the T-shirt. More thoughts.