Interesting, didn't know that.
Yeah. Indeed, is it OK to reference Anatoly Karlin here? He has a nice graph (in Russian) about this, but I first want to get permission to post this link since I don't want to get in trouble.
OTOH, the scenario you proposed avoids the Russian Civil War and communist dictatorship,
That is absolutely correct.
which means that there will be many more ethnic Russians running around.
Very true; however, it is also worth keeping in mind that ethnic Slavs weren't the only ones who suffered under Communism. Indeed, if I remember correctly, Kazakhstan lost 38% of its total population as a result of the 1930s forced collectivization.
In addition to that, the Russian birthrate may stay higher and fall much slower without an aggressively secularizing regime.
Were the Bolsheviks particularly hostile to large families, though?
Also, wouldn't this affect both Slavs and non-Slavs? After all, if both large families and religion are more widely accepted in Russia in this TL, wouldn't this effect also be visible on non-Slavs (including Muslims)? Basically, what I mean is that non-Slavic--especially Muslim--fertility could also decrease at a slower rate in this TL.
Not to mention the likely avoidance of a WW2-type devastation of western Russia.
Very true. Indeed, WWII appears to have hurt Slavs more than it hurt non-Slavs.
So the Muslim population of old and new territories would need to catch up not to OTL's number, but to a far bigger number of ethnic Russian than OTL's.
Very true; however, they would have at least a little help in regards to this considering that Kazakhstan wouldn't massively suffer from forced collectivization in this TL and considering that Muslim birth rates might remain high for a longer time period in this TL.
Then there's the issue of the conquests. Some of these are plausible, some not so much...and having all of them in one scenario?
If Russia has a sufficiently ambitious leadership, I could certainly see it trying to expand as much as it can. After all, Germany--its main land-based rival--already had its military power largely destroyed by WWI (and there's no Hitler and Nazis in power in Germany in this TL, so yeah).
Indeed, if Russia makes a major land grab in this TL, its main opponent would probably be Britain (France would probably rely on Russia to ensure that Germany remains down--and thus be unwilling to fight Russian expansionism elsewhere--while the U.S. probably still descends back into isolationism after the end of WWI in this TL). In the 1940s in this TL, Russia can very likely take on Britain. Plus, there
is a historical precedent for large-scale, almost non-stop expansion in the 1930s and early 1940s in our TL--specifically Fascist Italy, Nazi Germany, and Imperial Japan.
Even the motives wouldn't be clear for some of them.
Acquiring more natural resources and a better strategic position should be good enough, no? Indeed, Lavr Kornilov or whoever else is in charge of Russia in the 1930s and 1940s in this TL might view countries such as Iran as simply being a larger version of the conquests that Russia made in the late 19th century.
As far as "Lebensraum" goes, Siberia and the Russian Far East - with their vast tracts of land and natural resources - were already fulfilling this purpose.
True--as was Central Asia. However, this might not be enough for Russia--especially considering that Russia will have much more people (due to a lack of Civil War, et cetera--as you yourself have previously mentioned) in this TL. Indeed, even though the U.S. already had a lot of living space, it still started a war with Mexico for additional living space in the late 1840s in our TL. Thus, why exactly can't Russia do the same--but replace Mexico with, say, Iran or Afghanistan?