Oof where to begin. Besides what Arakadia already touched on, I'll elaborate:
So, she didn't actually "drop a cup size" as you've put it. I only slightly shrunk ONE of her boobs to give it a lil more depth (since she's turning away, and, y'know, things look smaller if they're farther away) and make the shape more feasible. I didn't even touch the size of the other one lmao
I understand that they were trying to make her very thin, but I also understand that no person actually looks like she does shape-wise, so I figured I'd have a go at making her body more realistic.
I have no personal issues with whoever drew this, as I know they were probably given a prompt and followed it. But I do agree that having a character whose body type can be best described as "starving" seems like it might cause harm in the long run.
I can dig a chiaroscuro drawing as much as the next person, but it's all about knowing where exactly to put the super dark shadows. In giving her a little more body fat, I also decided to try and match everything depth-wise as much as I could. Just going off of what my own clavicle does, I don't think the amount of shading is quite accurate even for a thin person.
I think I just about covered everything you addressed, let me know if you have any more questions. Not every day you get to hear a girl tell you how the female body actually works ;)