Historically Slavs were only a small part of Europe's population

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Today Slavic-speakers are perhaps the most numerous linguistic group in Europe, at least if you include Russians. But this only really became a reality in recent centuries. In the Early Middle Ages Slavs lived over vast territory but population density was low.

Population estimates for several areas in the Early Middle Ages:

France in 1000 AD: low estimate 6.5 million, high estimate 9 million
Iberia in 1000 AD: low estimate 4.6 million, high estimate 9 million
Italy in 1000 AD: low estimate 5 million, high estimate 7 million
Germany* in 1100 AD: low estimate 10 million, high estimate 12 million

*Only "Altdeutschland" west of the Elbe - without East Germany, which was a Slavic land back then.

On the other hand, for Slavs I found the following estimates:

West Slavs in 1000 AD: low estimate 1.8 million, high estimate 2.5 million
South Slavs in 1000 AD: low estimate 1.2 million, high estimate 2 million
East Slavs in 1000 AD: low estimate 3.5 million, high estimate 4.5 million

TOTAL Slavic population in year 1000 AD: 6.5 million (low) to 9 million (high)

Poland in 1000 AD had 1 million up to 1.25 million, so around half of all West Slavs.

The remaining half of West Slavs lived in Czecho-Slovak lands and in East Germany.

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As you can see Germany, France, Italy and Iberia each had a comparable population size as all Slavic lands altogether.

I also have data about Slavic populations at the beginning of the 19th century. I will post this data below.

Even at that time (early 1800s) France alone had more inhabitants than the total number of ethnic Russians.

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Stanisław Plater in his book "Geography of Eastern Europe...", published in 1825, gives the following numbers:

East Slavs - 36 million (28 million Russians, the rest Ukrainians, Belarusians and 1 million Cossacks)
West Slavs - 14 million (including 8 million Poles)
South Slavs - 5 million, but he forgot about Bulgarians, we should add at least 1 million Bulgarians

TOTAL - around 56 million in the early 1800s (ca. 1820). France alone had over 30 million people at that time.

For ethnic Poles, I also have estimates for the 17th century and for year 1900:

1000 AD - 1 million
1600 AD - 5 million
1648 AD - 6 million (it then declined due to wars in late 1600s & early 1700s)
1800 AD - 8 million
1900 AD - 20 million (including Diaspora, in the USA already ca. 1.9 million)

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Today Wikipedia estimates Slavic population worldwide to be 360 million, this includes Diasporas outside Europe:

https://en.wikipedia.org/wiki/Slavs#Population

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It is interesting that around year 1000 AD South Slavs were ca. 1/5 (18-22%) of all Slavs.

Over time their share has declined to around 1/10. The share of East Slavs has increased.

In 1000 AD East Slavs were only slightly over half of Slavs, in 1800 already around 2/3.

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Also estimates for some other areas (estimated population in year 1000 AD):

Britain and Ireland - 2.5 up to 3 million
Norway and Sweden* - 1 million together
Belgium - around 400-600 thousand

Compared to France and Germany, Britain was a small population back then.

*Ancient Scandinavia: https://anthrogenica.com/showthread.php?15440-Population-size-of-Ancient-Germanic-Tribes