Here my comment on that...
You´re are the ever the loveliest chubby girl in the world.
You´re are the most beautiful chubby in the world. (perhaps "the little chubby girl in the world.") at least if she is this age. >>>http://dailypicksandflicks.com/wp-conte ... -meme-.jpghttp://legacy-cdn.smosh.com/smosh-pit/0 ... girl-2.jpghttp://28.media.tumblr.com/tumblr_kpowg ... o1_500.jpghttp://i2.kym-cdn.com/photos/images/new ... 72/d56.jpghttp://i1.kym-cdn.com/photos/images/new ... 274730.jpghttp://shivisays.files.wordpress.com/20 ... .jpg?w=640http://i3.kym-cdn.com/photos/images/ori ... 38/465.png
As for she taking offense or not, I don´t know; but if you think she may or may not take offense, perhaps is not the best time to risk knowing it firsthand.
And maybe you can "weaken'' the statement by saying it´s just a nick, in this way:
You are the most beautiful "chubby" in the world.
Well, now I leave to more advanced learners.
It´s worth noticing that sometimes it´s interchangeable the meaning of chubby
as chubby-cheeked (a chubby-cheeked baby, for instance.) or chubby in general (a chubby baby, with chubby legs, arms, cheeks etc).