In Bioshock 1 the little sisters where not as big as a part as
they are in Bioshock 2. in Bioshock 2, the little sisters aren't as
ugly so you develop a relationship with the little sisters, because
your a big daddy, so you want to protect and save them. Plus,
Bioshock 2 is set after Bioshock 1, so the little sisters are
older, making their bodies more developed.