I think 6lbs is small. She must be a small cat. Sunny is a small cat. Jack is a big tom so I guess i'm used to seeing him, and he's a fat lump of a moggy!
You're supposed to able to feel a healthy cats ribs, but they mustn't be showing, and neither should their spine show. You should be able to feel their shoulder blades but they shouldn't stick out.
She is probably a small cat, but as long as she is eating and is healthy eating what she wants she's fine.