Catherine appears to be very healthy, happy, and athletic. She has a slim but muscular build and is clearly very physically fit. Her face betrays none of the signs of restricted eating.
Although sadly not her height, I have a similar physique - slim and athletic. I swim, run, and do yoga regularly, go for long walks with friends/dogs etc. I'm also very busy but I eat a very healthy diet, I never go hungry. I'm sometimes surprised by how much other people eat. If I go out to lunch with a girlfriend, I might eat one piece of bread, but I notice others will often finish the basket and ask for more. It's just a different habit and attitude to eating.
I've noticed that people who are larger, who don't do as much exercise or who eat more tend to assume that slim women have some sort of eating disorder. It's actually perfectly normal to be slim. 😊
My mum is 87, incredibly fit and active, very happy - and has maintained her weight at an even 8 stone since the 1960's.