Do you have a photo of them next to each other or can you get one? My Black shoulder hen always looks like the biggest peahen to me, but then when I see another hen stand next to her, I realize she isn't really bigger or at least not by much. I think there is something about that black shoulder color combination that tricks your eyes into thinking the peahen is bigger than she really is.
By bigger do you mean height or body size? Green peafowl are usually thinner, but yes greens and Spaldings are usually taller. Thora isn't very high % so maybe she didn't get those tall genes at all. Marshmallow might have better genes for height. Is Marshmallow the dominant bird? Lower ranking peahens can sometimes look smaller than they actually are, because they don't stand up as high when they are around the dominant peahen.