Part 1

So, first of all, the numbers in the thread you cited are higher than the numbers I gave in my comment. The map on /r/europe gives the total number of Jews in Europe in 1933 as 9,490,600 while the numbers I have given amount to (as it states in the table) 8,861,800 people. So, I don't know where you get a supposed "increase" from the numbers in the europe post to mine from.

The discrepancies between the two tables can be explained through a variety of factors:

First, my table with the lower number deals with fewer countries as Britain, Sweden, Turkey, and Spain are not included. Rather my table focused solely on the countries where the Nazis actually murdered people (though it is missing North Africa since that is a bit tricky).

Secondly, the info in my table comes from Wolfgang Benz Dimension des Völkermords. Die Zahl der jüdischen Opfer des Nationalsozialismus was published in 1991 when in some cases access to Eastern European archives was difficult. This leads to a discrepancy in some numbers, specifically Yugoslavia. There Western scholarship assumed a lower number of Jews living before the war due to the fact that Titoist policy tended to overestimate the number of victims. Additionally, newer scholarship had to correct the Soviet numbers to be higher since their archival material has become available, which it was not in 1991.

Thirdly, the point in time and the break-down in numbers. While the europe numbers deal with the Soviet Union as a whole, my table breaks it down into the Russian SFSR, the Belorussian SSR, and the Ukrainian SSR, which means that e.g. the Soviet Central Asian Republics, where the USSR evacuated/deported a lot of Jews from Russia and Eastern Poland are not included in the numbers. Furthermore, while the europe table deals with 1933, my breakdown deals with 1939, where issues such as immigration and border changes influenced the numbers in ways like Hungary's Jewish population going up while Romania's going down or France having a significantly larger Jewish population due to Jews fleeing Germany to France as well as German emigration in general.

Fourth, the question of who is Jewish. The problem here is who is counted as Jewish – people who self-declared as Jews because they practiced the Jewish religion or people the Nazis declared to be Jewish because they had two or more Jewish grandparents?

In the latter case, you'll get a significantly higher number of Jews in Europe. As did the Nazis btw. who themselves had a bit of trouble in determining just how many Jews there were. Just look at the Wannsee Protocol of early 1942 where the Nazis give their numbers as follows (p. 5):

Germany proper 131,800

Austria 43,700

Eastern territories 420,000

General Government 2,284,000

Bialystok 400,000

Protectorate Bohemia and Moravia 74,200

Estonia - free of Jews -

Latvia 3,500

Lithuania 34,000

Belgium 43,000

Denmark 5,600

France / occupied territory 165,000

unoccupied territory 700,000

Greece 69,600

Netherlands 160,800

Norway 1,300

B. Bulgaria 48,000

England 330,000

Finland 2,300

Ireland 4,000

Italy including Sardinia 58,000

Albania 200

Croatia 40,000

Portugal 3,000

Rumania including Bessarabia 342,000

Sweden 8,000

Switzerland 18,000

Serbia 10,000

Slovakia 88,000

Spain 6,000

Turkey (European portion) 55,500

Hungary 742,800

USSR 5,000,000

Ukraine 2,994,684

White Russia excluding Bialystok 446,484

Total over 11,000,000

And while they say this only includes those who practice Judaism, they still seemingly produced an overestimation of the Jewish population of Europe.

All that said, however, all the number magic of Holocaust deniers can not conceal the fact that number of people killed in the Holocaust, ranging between 5.4 and 6.2 million dead Jews, is not solely dependent on how many Jews lived where. While this is part of the evidence, a lot of of proof for these numbers comes from the perpetrators themselves, as I have written about before in-depth.