Alice is the star of a new series aimed at younger girls, so she's given a smaller chest to appear younger to help her relate with the target audience.
Alice is the star of a comic in a women's beauty magazine. Alice's experimenting with supposed methods for a bigger chest serves as a medium for the author's advice to women regarding that subject.
Lampshaded: Alice looks down her shirt, and says to her breasts, "Where are you guys?"
Invoked: The very busty Bobbi routinely flaunts herself around Alice to stir up feelings of inadequacy in Alice.
Exploited: Bobbi, of the Invoked example, notices Alice's preexisting angst, and flaunts herself to bruise Alice's self-esteem further, knowing that she would ask Charlie for a date as a measure to reinforce said bruised self-esteem. Bobbi does this because she knows Alice and Charlie would make a nice pair.
Bobbi realizes she's going to meet the flat chested Alice, and having some jealousy inducing assets, chooses to incorporate a sweater into her everyday wardrobe to help smooth over their acquaintance.
Bobbi tells Alice that Alice is actually a B-cup, not an A, and her breasts are actually only slightly below average - and that many women are actually, because of public perceptions, wearing a bra a size too small.
Discussed: Two characters are discussing, "It's sort of funny how ladies tend to get so flustered over something as insubstantial as their cup size."
Conversed: Two characters are discussing a work they like, "Well, at the very least, I guess we should be happy they included a female character who doesn't look like she should have back problems."
Implied: If you look carefully you can see a character with a flat chest sometimes staring at buxom girls or looking down at her breasts when well endowed girls are around.
Alice is eventually Driven to Suicide because she's so unhappy with her lack of assets, and partially prompted by Bobbi who frequently bullies her about it.
Alternatively, every single relationship she tries to get into fails because she can't look past the fact that she has a small bust. Guys that would ordinarily be interested in her are turned off by her extreme angst.