Physical ideals have been around for a while - witness the physiques on display in hellenic/hellenistic statuary.
Right. And that culture's sentiment on beauty is represented in those statues. This Hellennistic relief shows their notion of the idyllic female form with a few more pounds on the hips than our culture's most popular representations. A millenia and a half later we see this:It seems the plumpness of the idyllic female form is inverse to the availability of food to the general populace.
In our "Super-Size" culture of plenty, we herald the shapes of Tyra Banks, Halle Berry, etc., as the pinnacles of form.
Were the women depicted by Titian really representative of the ideals of his time? If the ideals depicted in his paintings were truly widespread it would seem as though you'd see them embodied in the works of more than a single painter, and you'd have dozens and dozens of paintings, frescoes, etc out there idealizing women that check in at two bills. Perhaps this is the case, but the fact that Titian is the one and only artist that comes up when people claim that at one time, a copious amount of adipose tissue was the sine qua non of the female form makes me suspicious.