3d models bigger that irl size?

I’ve thought about what I want my dick to look like when I’ve reached my PE goal, so I went and purchased some plasticine(like playdob but more stiff), so I can model it any length and girth. But when I measure the girth I’m now and I model it to be exactly that, the 3d model feels and looks bigger that my dick. Why would that be? I get it that there isn’t a fat pad in the 3d model, but for girth, 3d model is thicker than my dick at the exact same measurements? I’ve done several measurements and still? Is it an optical illusion? Even 4 inches girth feels and looks thicker than me?