All collectibles are idealized to a certain extent, especially this particular line. Would Willow in high school wear something like that? She did on Halloween, almost that costume exactly, only more revealing. Granted, that's not what is depicted here, but it's not accurate to say the character didn't dress that way. And later in the series she frequently dressed more provocatively. One could look at this as kind of a pastiche of the character throughout her existence - it's a combination of the high school Willow with the more confident uber-witch of later years. Buffy never sat in a chair like the first maquette has - it's more a overall representation of the character from throughout her run.
I don't think the bust line is unrealistic or an exaggeration either. To be frank I never would have even dreamed that their could be a complaint that this is an over sexualized version of the character and I'm sure the question didn't even come up during development of it.
This line is clearly a fantasy line, not any canonical representation of Buffy or Willow or Faith (from the shadow pic). Buffy never sat on a demon throne and Willow never wore that outfit and never had a werewolf teddy bear. In looking at the preorder page more closely, Trevor's headsculpt is lovely, but it's not Alyson Hannigan. Buffy is not SMG. It's clearly not a literal translation of the show. I know that's how you have to approach it in order to not drive yourself crazy obsessing about what could have been if the Whedonville franchises still had juice and manufacturers were fighting for rights to it.
However, these are Buffy pieces and have to maintain some relationship to the show, right? It seems pretty unambiguous to me that this Willow is a season 3 representation, due to the hair and the werewolf references. Up to that point, Willow certainly did not dress that provocatively. It wasn't in her character to. Her bizarrely un-stylish and at times mother-dressed-me-today outfits were a part of her character. The whole point of the Halloween episode is that the prostitute outfit was a costume--be someone you're not for the night. Okay, at the end, when she survived, she ditched the ghost overlay and walked down the street in the hooker clothes, but that was it.
I'm still in the middle of season 3 in my marathon right now, but from what I remember of season 4, she wore a lot of tomboyish outfits. Then after she came out as gay, season 5 and 6, she (and Tara) started wearing earth-girl outfits. Maybe I'm not remembering correctly, but I don't remember her ever wearing a miniskirt without tights. That tells me that part of the style Sideshow was going for was to sexy her up a bit. Why they did this, I don't get because it wasn't Willow. Up to season 7, she had to ask (Kennedy) really? when she got hit on.
Whatever. It's Sideshow's concept of Willow. Folks who are aligned to that aesthetic and don't mind straying from the original will like it. Those who don't will either have to compromise or wait for something better to come along. I think the reason I am so disappointed is that I don't think another opportunity will present itself for this franchise. So shoulda woulda coulda.