-The Y Zone

Depth & Lore Explanation
The shape of  the Y Zone is actually all the same width all the way from the belt to the collar, however the material of this jumpsuit is very stretchy,  so when you put it on a body like male for example, it will stretch according to his proportions. which is why in the "Temple of Trials" when you see the jumpsuit it is the same width all the way through, but when the character wears it the proportions of the Y Zone Stretch, so without being said it is a lot smaller then it actually appears and may only look loose because it's just hanging. This is canon since Vault suits are easily produced for large number of people, being compatible for male or female, and are known as "A form-fitting suit model manufactured as single piece coveralls from extruded rubber,"  so therefore we can draw the conclusion on why there are inconsistencies with the Y Zone.

References

NOTE 
I traced female too but I lost the image, just gonna have to take my word for it