Pettanko
The pettanko is a Flat Character. Literally.
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"Well, excuse us for being flat!"
—Etna and Flonne, Disgaea
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This might actually bother her if she is already considered childish and prone to tantrums, so the sore spot becomes endearing. Occasionally, she is proud of that flat chest as a way to appeal to a certain demographic of guys, although real fans in practice make a distinction between merely being flat-chested and actually being underage. Nonetheless, it is very useful in creating a character who looks younger than she actually is.
Also known as "Delicious Flat Chest", or DFC, a term apparently coined by Image Boards.
Now has its own song.
Contrast, obviously, Big Breasts Big Deal (and its subtropes).
Note: This is a fan-speak term, not a trope. Do not use it as such. Doesn't mean it can't still be used, but just not as a trope.
For actual tropes with examples, see A-Cup Angst and Petite Pride.