Over the past few years, a new-ish character archetype seems to be emerging in manga. As far as I know, there’s no widely accepted term in Japanese or English, but the two common words used to describe them are kyonyuu, or “giant-breasts,” and ponkotsu, or “piece of junk.” The former word is pretty self-explanatory, but the latter likely requires some explanation.
A ponkotsu character is described by the Pixiv Dictionary as a type of moe female character who seems cool and capable on the surface, but is a comedic wreck on the inside. Two examples of ponkotsu characters in recent memory are Kawashima Momo from Girls und Panzer, the student council vice president who tends to panic in high-pressure situations, and Aqua, the “useless goddess” from KonoSuba! In fact, one might argue that all of the characters in KonoSuba! count.
The small trend, then, seems to be pairing the ponkotsu type with a large chest. I’ve found who qualify for that criteria are Shidare Hotaru from Dagashi Kashi (above), the titular character from Magical Sempai, and Takizawa from Bijin Onna Joushi Takizawa-san (both below).
As heroines, “busty failure” characters appear to share many of the same physical characteristics and mannerisms, even when ignoring chest size. In particular, their facial expressions seem to exist on a spectrum ranging from “incredibly smug” to “profuse blushing,” with these characters most commonly falling somewhere in the middle. Also, they’re frequently incredibly intense individuals.
Given their beauty and their curvaceous figures, there’s an obvious sex appeal component to the archetype. What’s unclear is why this exact combination has taken traction, in contrast to the standard moeblob, e.g. Asahina Mikuru from the Haruhi. For example, clumsy dojikko types are a dime a dozen, but there’s plenty of characters of all chest sizes who fall under that umbrella. Perhaps there’s something fascinating about having these girls be, in a sense, “mentally clumsy.” Maybe it’s that having these girls be “perfect” physically provides a powerfully arousing contrast with how easily flustered they are.