Tell her your worried about her weight because she might not see your grandkids one day. She'll have no clue you want her to be sexier.
Watch a shit ton of chubby porn. There's no escape for those of whom wish to fix a problem with a two minute conversation. Or be active yourself. Ya know, like people were doing for eons before we all became lazy, and ask forums for advice instead of just being honest?
This might be something to approach as a couple and maybe not too explicitly. Think about coming up with a physical activity that you both can enjoy and do together. Figure out what kind of food is healthy for both of you and that you enjoy. Don't try to get her to do anything you won't also do.