Slightly chubby, but not enough to be 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.
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