Although he looks like a mature animal (see FunkyGibbon's full body photo below), its probably due to being overweight for his age- he was probably only about ten years old or thereabouts at his death.
Agreed. I have several old postcards from Berlin Zoo depicting ‘Bobby’ and I’ve seen other photographs of the living ‘Bobby’ in various publications; the taxidermist did a good job of capturing his facial characteristics, the mount looks very much like him.
(Incidentally, since I took this photograph, the exhibits in the museum have been re-arranged; on subsequent visits I found it almost impossible to take a photograph of him without getting lots of reflection from the exhibits opposite.)