"Rubenesque" comes from the art of the Flemish painter Pieter Paul Rubens, although just about every other baroque artist featured the same body types. Most likely his then-popular series of twenty paintings featuring King Henry IV and Marie de Medicis (along with several other women) was the source of the term.
The term is an adjective for a particular female body type. The closest synonym would be "voluptuous," or perhaps "curvaceous," but those words tend to refer exclusively to a woman's breasts. "Chubby" is wrong, and is in any case pejorative, where "Rubenesque" is not. "Full-figured" is probably accurate, but these days that's just a euphamism for "chubby." The best way to explain it is simply to point a finger to some suitable artwork, but as E2 is a text-only medium, I shall strive to do my best.
A Rubenesque woman is not "slender" or "athletic." She's not "fat" or "overweight." She's certainly not "obese." She has some body fat on her, but only as much as was considered healthy and appropriate in Rubens' time. She does not suffer from heaving cleavage. She is still very definitely shapely, an hourglass figure--the hourglass is just a bit wider, that's all. The word is not an insult. It is simply meant to be accurate and, hopefully, flattering.