I don't see
Abby as just short for
Abigail. I really dislike
Abigail and the one
Abby I know would never introduce herself by her full name...I'm actually not completely sure
Abigail is her full name. And if you're honoring,
Abbey's great.
Lily is not just short for
Lillian. First and foremost it's a flower name. That's like saying
Rose is just a nickname to
Rosalind, which is also a name not even related to the flower meaning.
Katy and Maddy are too insubstantial alone in my opinion. There are a lot of full names you could choose for
Katy. If you want to keep it short and simple (which I think a lot of people don't take into account when they criticise people for using nicknames), use
Kate.
Kate is traditional, simple, and sweet. With Maddy, you have some very pretty options for full names:
Madeleine,
Madeline,
Madison, and my favorite rarity, Madelieve.
My name is
Amy and it's a full name in it's own right, but you'd be surprised how many people think it should be short for
Amelia. Doesn't mean it should be. They're totally different names. I guess it would be nice to have
Amelia to fall back on, but I think I would have trouble deciding which one to go by because both are socially acceptable. Same with
Abby and
Abigail to me. I'd actually take someone more seriously named
Abby than
Abigail personally but that's just because the name reminds me of a comical milk maid. I don't know why.