It is a little confusing, I know. The seven largest ships are usually referred to as Grand Class. Of these, Grand Princess, Star Princess and Golden Princess are sisters with a capacity of 2600. Caribben Princess, Crown Princess, Emerald Princess and Ruby Princess are nearly identical to Grand, Star and Golden except there is an additional deck of cabins (Riviera), and Skywalker's Nightclub does not hover over the stern like a giant spoiler, except for Caribbean Princess, which kept Skywalker's all the aft. Caribbean's capacity is largest at 3100; Crown, Emerald and Ruby are only slightly less at 3070.
Diamond Princess and Sapphire Princess are sisters and sort of an amalgam of the two other styles--there is no Riviera Deck and Skywalker's is not all the way aft. Their stated capacity is 2670.
Island Princess and Coral Princess are sisters, and completely unique from the other large ships. The ambience and furnishings are very similar but the layout is different.
The Caribbean, Crown, Emerald and Ruby were built with the Piazza. It has since been added to Star and Golden, and soon to the Grand. Island and Coral do not have the Piazza per se, but their Atrium Plaza is much the same idea.
I hope this helps a little.