Can we have a bit of an exploration of what “realistic” numbers for 40k would be - to get an idea of just how much they would distort the setting?

One thing I see asked a lot is what a “realistic” scale 40k would look like. Population is the quick example:

Terra’s population = Several hundred billion

=~ 2000 people/square km

= about 2/3 as dense as Austin

The retort which comes up quickly to end such discussions is that “real scale” would distort the setting.

Hard to care for one space marine if realistically there could be billions ... I guess?

So ... just how much distortion are we talking about here? Which parts of the setting just wouldn’t work if they tried doing the maths on the figures?