Yep they are called dwarf platies. I got into a big argument with a moderator on another site about whether or not dwarf platies are an actual species of platy, or just a mutated version of a normal platy. [/quote]
They are nice colourful fish.
Don't wish to retread old ground, or to stir up a hornets nest, or to split hairs; but I just wanted to point out that they are not an 'actual species of platy', especially if they are a hybrid as claimed. I will go for the fact that they are a variety of platy. Xiphophorus variatus is a species, sunset, tuxedo or dwarf is a variety.
I am happy to buy the fact that they are a separate variety, especially if they are breeding true. Many species of Xiphophorus including Xiphophorus nigrensis and Xiphophorus milleri produce small morphs and often these smaller morphs are more colourful than their larger counterparts and it is possible to line breed for various traits, including small size. Maybe it is possible that in this case the smaller morph is more colourful, like in their wild cousins.