Tail shape and size is something a lot of breeders breed towards (while some go strictly on colour and pattern). A huge tail probably is the result of many generations of selective breeding. If his tail is always fanned out and undamaged, he's a happy, healthy guppy.
Yes guppies nowdays come in many forms and as said this is due to selective breeding. Nothing to worry about Mind you so are Betta's (Male) in the home aqauria but in the wild they are quite dull and have fins like a female.