Not every anime character is happy about their appearance. One common reason for female characters to be less than enthralled by their appearance is breast size. Those who don't love their own look may find themselves getting jealous of the people around them.
Most anime girls with breast envy are flat-chested girls who wish they could be like their more well-developed friends. Whether it's Kaguya of Kaguya-sama: Love is War refusing to go to the beach for fear of being compared to the busty Chika or Louise from Familiar of Zero screaming about how big-breasted women are the enemy, this trope is not at all uncommon. But there are also well-endowed anime girls who wish their breasts were smaller - Himawari from Yuru Yuri would gladly shrink her chest if she could.
These anime girls may not like their breasts, but that doesn't mean we don't like them! Vote up your favorites and show them a little love.
Sakura and Hinata both have breast envy, but for completely different reasons. Hinata has always been busty, a fact which embarrasses her. Considering how shy she is, it's not surprising that she doesn't love having breasts that float upward when she's trying to relax in a public bath. Meanwhile, Sakura is self-conscious about her flat chest and isn't exactly ecstatic about Hinata's lack of appreciation for her gifts. Hinata isn't always understanding of Sakura either - in fact, an alternative universe version of Hinata openly mocks Sakura for her small size.
These moments are anime-only, though, so if you'd prefer not to think about these powerful kunoichi being less than body positive, you can always consider this non-canon.
Because she's a a generally envious and competitive person, it's no surprise that Kaguya Shinomiya has some less than gracious feelings about her fellow student council member's chest size. While Kaguya's proportions are modest, Chika Fujiwara is extremely well-endowed. Kaguya is desperate to make sure that the student council president, Miyuki Shirogane, doesn't get distracted by anyone else's assets. For this reason, she vetoes a trip to the beach that would involve both herself and Chika dressed in swimsuits, highlighting their different figures.
Most of the time, Kyoka Jirou's focus is on her goal of being a hero and her love of music, but once in a while, she feels a twinge of jealousy when it comes to her breasts. Unlike every other girl in Class 1-A, Jirou is almost completely flat-chested. When Mineta, the resident class deviant, gets excited at the prospect of spying on every single one of his female classmates except her, Jirou doesn't exactly want Mineta to harass her, but she isn't thrilled about being excluded, and it's implied that it's because of her chest size.
According to Emi Yusa, small breasts are a good thing, because they don't get in the way of combat the way big ones do, and she doesn't have to spend more money on armor. This is little more than a rationalization - actually, Emi really wishes she were much more buxom. This is blatantly obvious from her negative reaction to Chiho's comically large breasts.