It isn't that they all have to have it.
It's just that if I'm gonna believe she has power she's better be packing something grade "A".
She's one of the main characters, she needs more design.
And no I don't want whatever her name is from bleach with the abnormally, disturbing, over-sized, Monsters on her chest used for fan service, even though it's more likely to kill a boner.
According to you, from what i got if a female character is strong she needs to have big tits? Several characters in different Manga/Anime have a great design and guess what? they don't have cow udders.
Sakura's character is well done She is strong without all that unnecessary oversexualization, Many female characters who have a lead role with big *** tits usually get used for fan service examples are Orihime and Lucy and guess what? They tend to be useless over 80% of the plot (Mary sue) again Orihime and Lucy. There are only a few ones who are actually useful in the story.
But damn get that requirement out of your head it's bad, whenever i watch an Anime and i see cow udders on a character i just cringe at what she is going to get used for later fanservise(Which actually happens most of the time). While the flat counterparts are actually useful for the plot.
So summed up Flat Chested + Great overall design = Great character and Cow udders + bad overall design = Terrible character.
