I don't have quite the same idea of these 3 words as some of you.
To me, rubensesque (as I write it) denotes well-endowed and welll-upholstered women (if you look at a Rubens painting you get the idea) but no so much so as "portly" or "stout". In my vocabulary, "stout" is a euphemism for plain old fat, or obese (but not excessivly so), and was so used by my grandmother who was herself stout. I would not apply "portly" to a woman. To me, "portly" is applied to a man who is stout but also big in general with an impressive carriage, like J.P. Morgan or King Eddie 7, but not to the point of looking like W.H. Taft.
In my view, and as I use it, "matronly" does not necessarily have anything to do with size, although most matronly women are bigger than average. It has to do with age, also. A young woman who looks matronly is indeed a sorry sight. A perfect example is Queen Lizzie Twoth. She's not fat, nor ancient, but her clothes, carriage, & general stiffness of affect is what makes her matronly in my view. The greatest example I have ever seen was the mother of a friend of mine, who was in her 50s, somewhat heavy but not excessively, always very conservatively dressed in long dresses and carefully corseted, old-fashioned piled-up hairdo and a very dignified carriage. When she walked into a room it was like the Queen Mary coming into the dock.
