According to statistics publicized by BMDW (Bone Marrow Donors Worldwide), not only is Ezer Mizion the world's largest Jewish bone marrow donor registry, it is also the fourth largest registry in the world. 

However, this data, impressive as it is, tells only a small part of the story.  If we look at the number of registrants in each country, compared to its total population, Ezer Mizion's Registry leads the world with the largest number of per capita registrants: over 5%. That means that more than one in every 20 people in Israel has joined Ezer Mizion's Registry.

In comparison, the per capita statistics in other countries are: less than 4% in Germany, 1.71% in the USA, and only 1.2% in the UK.  Italy's Registry, which has slightly fewer registrants than Israel, represents only .58% of the local population - proportionately about one tenth of Israel's Registry membership!

Sources: Population Center Country Index www.mnsu.edu/emuseum/information/population
Bone Marrow Donors Worldwide www.bmdw.org