It's interesting how recollections vary.
In my view Diana was always blocky.
She had a defined wast when the outfit was well chosen or when one of these belt(elastic) combo were worn(trendy back then)
The statues outfit implies that the shirt is not form fitting, so it's a bit out of the skirt, like men used to wear their shirt/trouser combos in the past (shudder)
Anna Harvey the Vogue editor was her friend, her sister Jane was assistant editor, and Diana had unlimited budget and much less scrutiny than Kate for example today, so it was easy looking fashionable.
It was mentioned then, that she had either a boob lift or small implants at a certain point, so some will probably remember the different stages.
Also it's a statue, it lacks "texture" "animation" and the kids somehow create depression and staticness as in usually children (unless a tragic depitiction) add life and dynamic effects to a scene, regardless if painting or sculpture.
It might be also the fame effect, many famous people are very
average(technically) their symmetry, proportions, golden ratios, etc are normal, then we are told by the media and their PR how perfect, and stunningly beautiful they are.
Humans being humans agree with it mostly because if they disagree someone will call them hater, so it works.
It's like Peg, and the creation of her, via the standard plastic surgery application of the phi mask changed her default features into (beautiful).