Caymans are sized by individual sizes - only whole sizes, not half. They still run a bit large, but not as wide as the Beach style.
Beach style is sized so that in XS, S, M, L and so covers 2 whole sizes per shoe. For example, the M fits a Women's size 8-9. Therefore, they do run a bit large, especially if you are on the smaller end of the size range. I normally wear an 8 1/2 (mostly due to width - I have wide feet), and I have a size 8 pair of Caymans which fit beautifully, and a pair of M Beaches which also fit well - but are definitely on the large size for me. I like that, but some people may not. Nothing drives me more nuts than seeing people's feet falling out of their shoes - EEEWWWW! I like to see at least 1/2 " of sandal bottom past my toes!