It's just some 2" 1018 I had laying around.
(Oops...looks like windows only. Mac guy here)
True, although since it is probably a D1-6 spindle, you won't find a direct mount chuck smaller than 8". You can mount a smaller chuck on a backplate, but again, the smallest backplate you can get is going to be 8". You can turn that down a bit (to 7-1/8") to match the spindle, and maybe turn it at an angle to match the chuck diameter (I have a 5" 5C D1-5 chuck mounted like that).For a large lathe there is no law preventing you from mounting a smaller chuck on the appropriate back plate to keep the weight to a manageable scale.