but the only one I can think of that includes the element of being somewhat overweight in its definition is rubenesque.

Rubenesque has as much of a right to wordhood as zaftig. Problem, for me, with matronly is that the word emphasizes the motherly or widows weeds: the state of pomp, not plump. Similar in fact to portly which seems to have been tied between stoutness and stateliness ... And how does one describe those chubby little putti that one sees in so much baroque art? Another word that comes to mind, in its etymological rather than its English meaning, is avoirdupois 'having some pounds'. While a zaftig woman can have an ample bosom, we wouldn't say that she herself, synecdochically, was amble, and in our saying it be kind in intent.

Good question, dxb. Perhaps somebody else has plumbed the plump and weighed the weighty.