Unlike the Japanese release, the character creator in the Western release Xenoblade Chronicles X doesn't allow you to change the chest size of female characters.
The English version of Xenoblade Chronicles X has been previously discovered to have a a small change when it comes to in-game character Lyn, with a change to her outfit to be more appropriate to her age.
The game's character creator also appears to have been altered, removing the ability to change the chest size for female characters.
Here's YouTube channel GameXplain showing the differences in both versions.
Xenoblade Chronicles X in out in December for Wii U.