Well I define curvy as someone who has a pear or hourglass shape (small waist, large hips and bust) and hence the sides of her body literally have curves because her waist goes IN and, in contrast, her other bits go OUT.
An overweight lady will probably carry some of the fat in rolls on her belly, hence her tummy will no longer go IN, but it will go OUT. Hence she will no longer have that hourglass outline and will no longer be classed as "curvy," at least, by my way of understanding it. She's no longer a pear; she's an apple instead.
Does that make sense?
I'd say the first model has a bit too much fat on her hips for her waist to be clearly defined and to fall into what I personally consider "curvy."
I do not wish to cause offence or imply that she isn't pretty. "Curvy" is only one type of figure. Many types are beautiful, but "curvy" is the one I'm analysing here!
Not saying I'm perfect either! Just observing what I see in the photos but not implying I look better than them! (Don't you wish. Ha ha)!
Of the four models below her, the one second from the left is the only one who has a significantly small waist in contrast to the rest of her torso. Her waist nips in neatly, in contrast to the other girl in the middle, who is rather "straight-up-and-down." The other two are hard to tell because they're not standing straight on.
I suspect that my definition of curvy is different to other people's, and when most people say "curvy" they just mean someone with big boobs, hips, or bum. I concur, but I add to that the importance of a small waist (sans corset) to set off the contrast. And I don't mean small in terms of measurement. I mean small in terms of contrast. The girl centre-right looks like her waist probably measures less, but it's very straight and hardly curved at all.
Loads of the Bravissimo girls get fake tucks airbrushed into their waists to nip them in for contrast and make them more curvy. I notice it a lot, because it's often quite clumsily done. They just photoshop in a pair of triangles. LOL