Here is a link: http://www.jewishvirtuallibrary.org/jsource/Judaism/jewpop.html
The global Jewish population increased from ~10,600,000 in 1900 to ~16,728,000 in 1939, the year the Holocaust really started. An increase of ~6,000,000.
After the genocide the population was tragically cut to ~11,000,000, and 40 years later only increased to ~12,868,000, an increase of only 1,800,000.
I hope this isn't too speculative, but what are some possible reasons for the slowed Jewish increase in population?
Some possible reasons:
Just general decreased birth rates (for the more recent years). It's no secret that Jews tend to settle in developed nations, as shown by the chart of where Jews are located. Developed nations have had greatly decreased birthrates (in recent years especially). Check out this chart for more information on birth rates by country, and you'll see what I mean. This doesn't explain the lack of population growth post-1945 considering that was the Baby Boomer generation, but you do get a feel for the recent history, I think.
Assimilation, as mentioned already, is a big factor. Many people who are Jewish are becoming assimilated and accepting other cultures as their own, or becoming atheist/agnostic/uninterested in religion on the whole. Identification is done through self-reporting, which means that as they assimilate they tend to no longer identify as Jewish. Some put "Decline to Answer", or just accepted some new religion/culture. For example (and this is just an anecdotal one to help you understand), my very religious uncle married a Christian woman (no, nothing wrong with that in my books, to be clear!). The children grew up celebrating both Hanukkah and Christmas, but they got presents on Christmas, had more family over on Christmas, and overall found it easier to connect to Christian holidays because of the fact that they learned English first (I learned Hebrew first, despite being born in the US, because I have Israeli parents who taught me before English, so I was bilingual by 4 years old). This is assimilation into the common culture, and it occurs especially frequently in the United States, where most Jews reside. Also, because being Jewish is typically regarded as being passed on through the mother, intermarriages tend to lead to the child being excluded from being "Jewish" by the religious, which makes it even less likely they'll assimilate into Jewish rather than Christian (or other) culture.
The chart cites its source, which actually gives a wonderful expansion on these explanations. The study can be found here, in case you're wondering. You can skip straight to the parts that talk about low birth rates for Jews, high intermarriage rates, etc.