I'd happily watch a slim Catherine over some pudgy fat princess waddling around any day. In all the royal houses there are very, very few who are overweight. Those who are, are hardly worth looking at from a fashion perspective and I love fashion. No one looks twice at Princess Charlene and her awful cover-up fashion, to go with her permanently miserable face. Same with the Dutch Princess, who also can never manage a smile. So the extra weight certainly doesn't help with confidence and a sparkling personality, unlike her mother, the fabulous Queen Maxima.
Princess Mary, Queen Rania, Queen Letizia, Princess Olympia, Princess Caroline, Rajwa Al Saif and many others always look amazing. Also can't imagine Charlotte Casiraghi representing Chanel while waddling like a duck either. Instead she gets her svelte figure on her horse and rides the Chanel show.
I may not like all of Catherine's fashion choices, but she'd look a lot worse if she were overweight. Of course the taller lean women with good proportions always look better than e.g. a Meghan who though now is slimmer again, still has that odd Sponge Bob figure. So slim is not always a guarantee to looking fabulous, but if you're prepared to tailor for the short squat ones, you can look great. Zara doesn't have a waist either, but she has a great stylist, who dresses her well to disguise that little flaw and she looks amazing.