Not a cat owner, just curious passerby. Where would you mark the difference between healthy and underweight? Or is it more of a thing you notice if they start behaving differently? Or is it something that just never happens?

My sister's cat used to always go outside and eat a mouse or something if she wanted more food than what they gave her (and sometimes just instead of eating her food, lol), so I wondered if maybe most cats are like that and so you don't have to worry about underfeeding them.