Hi Raoul

Triton 2706 km
Moon 3476 km
Io 3638 km
Callisto 4820 km
Titan 5150 km
Ganymede 5268 km...

...a spectrum more than "similar".

Actually since Callisto, Ganymede and Titan are ~ 1/2 water ice if you stripped them back to their silicates, the core would be roughly Moon sized, so you do have a point. The Moon itself doesn't quite belong because the core of Ganymede (and Callisto/Titan) would be like Io, which has an extensive iron-sulphur core. Our Moon has very little iron in its core.

Titan is different again because it retained a lot of ammonia according to Jonathan Lunine's work on its formation. The Galileans didn't gain nitrogen atmospheres like Titan because they formed too hot to get much, if any, ammonia which was then decomposed. There's considerable argument over whether any of the Galileans could have retained water oceans to the present day because water ice undergoes solid-state convection at low temperatures which very effectively chills down ice-crusts. Titan almost certainly retains and water/ammonia ocean that is colder than straight water and more viscous which fits in with its tidal evolution.

Triton is clearly an outlier - its density is 2.065 so it's more rock than ice, unless there's a lot of high density carbon dioxide ice (1700 kg/m^3 at 40 K.) Pluto is its nearest comparable object and it's pretty clearly a Kuiper object. Because of the strong tides Triton experienced during circularisation I would'nt be surprised if it lost a lot of volatile mass. Pluto also lost mass when Charon was formed, so the two would thus share that in common too.

Adam