Cockers can come in all sorts of shapes and sizes from pocket rockets to something closer to a prop forward! Rather than concentrating on weight, look at her overall shape. If her frame is nicely covered and she is eating well then unlikely to be anything to be concerned about. She may put on a late spurt of growth, or may always be small either way, as long as she is fit and healthy...
If in any doubt, get the vet to have a look.