your latter point about world size actually brings up an interesting aspect, even in a world where immigration was fairly rare by modern standards something like 250k jews fled every corner of the middle east to inhabit Israel.
but i'd still argue that jewish can in many ways be considered an ethnicity. mostly because of a history of displacement. kurds as goom brought up are the same, despite there being no kurdistan. hmong people would be the same as well, despite being forcibly moved throughout china, vietnam laos and elsewhere. i dont think a people can have a history of not being able to peacefully settle somewhere, and then denied the status of ethnicity as a result.
Of course it's an ethnicity considering for literal hundreds of years they lived as minorities in various countries across Europe and parts of ME and maintained their identity without being washed out. Very close nit group that married within and stuck together. A good contrast is Jewish groups and the US.
A bunch of Ashkenazi Jews lived in the USSR or a bunch of Sephardic's lived in the Iberian peninsula and maintained their Jewishness and their blood lines for hundreds of years. Contrast that to white people living in the US. Today most whites that live here that were born or their parents were born here have very little of old world culture. Drive through the Midwest and if you ask a group of white people they'll tell you they are Irish, German, Polish and half a dozen other things, but really any vestige of that old identity is long gone, they are just American 'white' at this point.