Some cats can have different body structures; While they have the same weight, some might look thinner and others might look bulkier. Also, there are cats that are thin as a part of their breed's description, such as the Devon Rex.
With age comes change and muscle wasting is common in older cats. Even healthy senior cats have less muscle mass than younger cats.
This might happen due to a couple of reasons: - They don’t like the food - Dental discomfort or pain - Difficulty to digest
Losing weight can be the result of some health conditions, such as Diabetes Mellitus, hyperthyroidism, cancer, Feline immunodeficiency virus, etc.
If there were some recent changes to your cat’s environment lately, they can cause stress and anxiety in her, which will result in her eating less and, as a result, getting much thinner.