On Earth there was furthermore a nasty nuclear war and I doubt that the population size is larger than today. It is probably smaller.
It has a population in the billions by First Contact.
And the casualties from World War III were in the 100 millions so it didn't kill the majority of the population.
For the first note:
Data: Population 9 billion, all borg.
That indicates the population with the Borg in full control, in a timeline where no wars post World War III took place, beyond replaced drones. This does not affect the normal/restored timeline population.
Funny, the movie seemed to imply there weren't really any wars after World War III in the regular timeline.
Also 9 billion is still in the billions.
And Riker didn't seem suprised at the 9 billion population count, whereas he was suprised about them being Borg. so I tend to think Earth had a high population.