The word pretty mean something very beautiful.
The word purty is something more pretty than pretty.
The word purtyful is something even more purty than purty and is used with more emotion; usually used in cute sentences to express great cuteness on something, for example used to express cuddly animals, stuffed animals, poems, drawings and things as such.
Sara: "OH EM GEE!!! It's so purtyful!!!!"
The bunny twitches its nose.
Me: "Yay! Must poke the purtyfulness! PURTY!!!!"