It is a formal way of emphasizing that your're beautiful in the eyes of the person that says it. Quite beautiful = very(above average) beautiful. It can also be used to express shock. If someone looks better than you expected them to look(for example online versus in person), you could use the expression - you're quite the beauty.

thank you julia! I'm happy then!

"Quite the beauty" means that someone or something is especially physically beautiful. Here are some examples: That racehorse is quite the beauty. Beonce is quite the beauty. So, "you're quite the beauty to me" is a compliment about your physical appearance in the eyes of the person speaking.