Looking around at definitions for envy though, envy is "resentment at what others have, that they themselves lack." Envy would then visually be depictions of anger and annoyance, they're upset at what someone else has. Envy would not include being sad or depressed, as it does not depict resentment at what another person has.
You can say that they're sad because of what other girls have, but you're missing a major defining difference. Notably there is a major visual difference, underlying emotional difference, and focus difference. When a character is envious, they're upset at what someone else has and their focus is on what the other person has that they desire. This is not the same concept as these other depictions of inadequacy, because they're not upset at what someone else has, they're upset at what they themselves lack and their focus is on themselves and their own inadequacies. The source of the issue might be the same, but the response is completely different. It is similar to reactions of being insulted, the response might be anger or it might be sadness, but we wouldn't label them all as "anger" just because the source of the problem is the same.
If we have to lump them all together, I would think "breast_inadequacy" would be the better term, as it isn't limited to depictions of envy and resentment and describes more the underlying source of the issue than the actual means of depiction.