Why is curvy associated with plus size/fat/thick?

A woman can be totally skinny and still be curvy because curvy means you have curves (boobs, ass, small, waist). And a woman can totally be plus size and not curvy.
Why is curvy associated with plus size/fat/thick?
To give you an example, in this photo the woman in the left is curvier than the woman in the right. You may say the right woman has bigger boobs, but she also has a bigger belly, which is normal to get when you gain weight. Have they been the same body fat% things would have been different. The woman in the left may not be the curviest, but she still has a more sculped waist and decent breast size for her current weight.

So this is curvy:
Why is curvy associated with plus size/fat/thick?
Why is curvy associated with plus size/fat/thick?
Why is curvy associated with plus size/fat/thick?
And this is not curvy:
Why is curvy associated with plus size/fat/thick?
Why is curvy associated with plus size/fat/thick?
Why is curvy associated with plus size/fat/thick?
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