i can tell you for certain that the neck isn't disproportionate in any way
source, ex gf of 3 years had the exact same looking neck/build


what i can say is that because of the hair it makes this neck seem much thinner than the anatomy suggests
you should've with out a doubt have had the bottom right side of the hair go behind her back rather than lay in front of it
seeing as how you have it darkened to assume depth that would've made it much more convincing and it wouldn't have helped this guy above me come to an odd conclusion

love the palette, dislike the shading but it's not incorrect seeing as those sparkles are multiple light sources, so oh well