Giant animals tend to be less diverse species wise, and the Nemegt sauropod remains were from the same formation, so the probablity that the Nemegt titanosaurs are from one species is a higher than for much smaller bird remains from differing sediments. As has been noted, there were 9 moa species just recently on wee islands, one proboscidean each in Africa and Asia.
How many proboscidean species you recognize in Africa depends -- surprise, surprise -- on your species concept. Many people nowadays distinguish the savanna elephant (*Loxodonta africana*) from the forest elephant (*Loxodonta cyclotis*).
And what leads you to describe New Zealand as "wee"? The fact that it's smaller than Texas?!