Commonly seen in anime and manga, less-developed girls are often envious of those with larger breasts.
Breast envy stems from the belief that "bigger is better" when it comes to a woman's breasts. Women with small boobs sometimes become envious, jealous, upset (or a combination of the three) towards those with big boobs.
One of many reoccurring concepts in many of the anime and manga franchises to be produced over the years, Breast Envy usually takes place when a female character with a small bust comes across another female character with much larger breasts than they have at the time. This usually results in the female character becoming jealous and envious of the other female characters due to the fact that their breasts are much larger than theirs and thus they are more likely to attract the attention of men, who seem to have a weakness for large-breasted women. It tends to be found more commonly in anime/manga franchises that have a ecchi and/or harem theme to them.
Most examples of this tend to be found in ecchi/harem-themed franchises, in which more than one female characters are vying for the love and attention of a male character. Fearing that they will be ignored or looked down upon since they do not have the large breasts of the other characters, the characters with smaller breasts grow envious of their busty competition and tend to go to great lengths in order to get attention from the male character in question.