oh how pretty 
how old is she? i don't want to alarm you but i had kind of the same problem with diotima (it was only 1 lb. but she's not a big kitty) and when she finally started losing (seemingly effortlessly) it was because her thyroid was 13.2. (it's supposed to be between 2 and 4).
any cat over 7 should have a baseline blood test. she had hers when she was 8 but didn't have this problem until she was 13. her level last time was 2.6, we're getting a regular check again on monday.
how old is she? i don't want to alarm you but i had kind of the same problem with diotima (it was only 1 lb. but she's not a big kitty) and when she finally started losing (seemingly effortlessly) it was because her thyroid was 13.2. (it's supposed to be between 2 and 4).
any cat over 7 should have a baseline blood test. she had hers when she was 8 but didn't have this problem until she was 13. her level last time was 2.6, we're getting a regular check again on monday.