When large is standard and the majority of female characters are big-breasted, usually with very few exceptions.
It should be noted this concept is not simply about a group of buxom women in a series. Anime and manga linked to this concept feature a female cast with the vast majority having big boobs, including protagonists, antagonists and even background characters, making it appear as though that is the norm for female characters.
This concept is often used in "ecchi" anime and manga as a major selling point. Sometimes, it's simply a personal preference of the creator, Fairy Tail being a prime example. Some anime and manga fans even consider "big boobs" an entire genre of it's own.
Even in a world of buxom women, there are sometimes exceptions, including the occasional Lolita or Pettanko character, and any female characters who have simply not yet developed. These characters in such an environment often stand out and may be teased by the other characters, leading to breast envy. However, sometimes a character's smaller size works to her advantage, particularly in harem-themed series in which the lead male prefers girls on the more petite side.
The Gainax Bounce concept often comes hand-in-hand with the World of Buxom concept.
Naturally, this concept is even more popular in hentai than in non-hentai series. Kyonyuu Fantasy is one key example.