Chubby. Between curvy and fat. A saying that goes around for plump is "Fat in all the right places".
The Japanese tags above are Pixiv's equivalent of the word, but be aware that not all Pixiv-centric uses of those tags fit Danbooru's definition. As with all other size-specific tags, observe the body and see if the tag actually needs to be used before adding it; make sure that the body is not simply “curvy” and that it’s not “fat,” either.
Plump is mutually exclusive with fat and obese.
The following tags are aliased to this tag: chubby (learn more).