On the left is how Ayane appears in NGII Σ and on the right is how she appears in NGII. As it’s been pointed out back when she was revealed for Σ, gone is some of the cutesy baby fat for thinner, more grown-up features. All of this, of course, makes total and complete sense!
The designer who worked on NGII is different from the designer who’s making NGII Σ. That, and time has passed between the two games. Maybe Ayane aged or went on a diet or something?