Well it is not good to say such words to anyone, as these can hurt the feelings of others. and one should not have a right to taunt or humiliate others by pointing out their physical appearance.
But if there is a case where someone has to use such words, then i guess Plump is the most positive connotation to be used instead of some harsh words like fat, dumpy etc. No doubt, Plump is also one of these words, but it gives a sense of cuteness as well. Because being chubby fat dumpy are always associated with ugliness. But by using the word Plump, some positive vibes are also being shared. So its better to use Plump, if necessary.