and from what I understand, it's also possible for the size of the placenta to be larger to compensate for dietary and environmental factors. i.e. smokers will often have larger placentas to compensate for the lack of oxygen, and people who are deficient in their diets will often have larger placentas to compensate. I have also seen references to the fact that sometimes it just happens, and provided all scary things are ruled out, it's not usually a cause for concern.
And yes, lack of protein can account for a larger placenta.