Great Mazinger
Mazinger Z usually looks shorter and wider compared to Great Mazinger, who has a more slender physique. This is mainly due to the "helmets", which is how one really can tell these two apart. The helmet of Great Mazinger is pointed while the top of Mazinger Z is "flat-topped". MazinKaiser looks more heavily built than either of them, and sports the typical over-detailed (more ornate) style of more modern mechas (consider there's a 30-year gap between the first two and him). MazinKaiser is also the bigger/taller of the three.