What I meant to say is that she's not skin and bones... some more
pounds than needed but, precisely because of that, she should be
actually more attractive.
Never say to any woman who is not already your girlfriend or wife that she is chubby. And even then, say it in a sweet, affectionate and gently teasing way so she won't burst into floods of tears and accuse you of being mean, heartless and insensitive. Some women will take a different approach and refuse to speak to you for the rest of the day or start reading the invisible list of faults you possess...
If a man had written this I would have have accused him of sexism but as it's me and I belong to the category of "not a skinny bird" I can safely say this.
Chubby is not complimentary; it means someone who is overweight, soft, round, and definitely on the heavy side. It is not the same as "in carne" an expression which I personally dislike, but for Italians it tends to have more positive connotations.
To compliment a woman on her figure because she is not "skin and bones" you could simply tell her she looks healthy. Not particularly exciting compared to curvaceous; curvy; voluptuous; full-figured (potentially on dangerous ground with this one); and "curves in all the right places". But if a woman is not thin and looks "normal-sized", (i.e. size 12 and above) healthy is a safe option.
N.B. There's nothing wrong with saying she is sexy and hot either, regardless of her body weight, but that implies you are sexually attracted to her and depending on circumstances it might be considered inappropriate or creepy and offensive at the worst.