My Hulk Maquete is definately heavier than Rhino. Rhino is fiberglass.
Personally I don't care. Hulk Maquette is heavy enough and Rhino feels like I could use him as a soccer ball and he would be fine.
Many times sideshow screw up the weight on the final product listing.
I've heard that Rhino was even hollower than the Maquette too! I've the Hulk Maquette and the weight is fine. I think that the main difference is that the Maquette is made of resin and Rhino is made of fiberglass, dunno!