I've never compared 2 side by side....but to me...Mollies have a rounder abdomen...look stockier than platies and have bigger mouths. I also find them not as nice looking as platies.
Mollies are much larger and longer than platies. They also have a wider mouth compared to platies. Look at my signature, I have 17 platies and 15 mollies and it's easy to tell them apart once you know the difference between them.
I never heard of dwarf mollies or platies? Maybe Fantasia is just older than the other fish. My platies range from a little under an inch to over 2 inches!