From World Jewish Population, 2010, a report by Prof. Sergio DellaPergola, published by the Berman Institute – North American Jewish Data Bank at the University of Connecticut:
I hadn’t realized what a large fraction of the Jewish population lived in the U.S. and Israel, and how little lived in other countries (such as Russia and Ukraine) that used to have a high Jewish population even after World War II. The data reflects what the report labels as the “core Jewish population,” defined as “all persons who, when asked in a socio-demographic survey, identify themselves as Jews; or who are identified as Jews by a respondent in the same household, and do not have another monotheistic religion.” The definition excludes “non-Jewish members of Jewish households, persons of Jewish ancestry who profess another monotheistic religion, other non-Jews of Jewish ancestry, and other non-Jews who may be interested in Jewish matters.” “If an enlarged Jewish population definition is considered, including non-Jews with Jewish ancestry and non-Jewish members of Jewish households, the United States holds a significantly larger population aggregate than Israel (about eight million compared to six million, respectively …).”
I should note that measuring such matters is difficult, for all the obvious reasons, and I can’t personally vouch for the accuracy of the report, though it looks credible.