Because it could just be referring to something other than their overall build, borrowing from a certain characteristic of rabbit anatomy... stuff that they don't really want to make any more explicit than that. I doubt it will tie into anything to do with "androgyny".

As for the concept art...



...is not miles away from...



Particularly not the second one.

He was using those words to set them apart from Hrothgar. It does not mean they will not add any bulk to a modified Elezen model. You can retain a slender build even with more bulk than is on the Elezen model, and certainly with more muscle definition, i.e. by making them more ripped.