BoS - around 500 to 600 members with 48 veritbirds and decreasing by the day.
Institute - 700 -1,000 synths and increasing by the day.
Raiders - 10,000 to 15,000 and increasing by the day.
Super Mutants - 9,000 - 13,000 and increasing by the day.
Feral Ghouls - 7,000 - 12,000 and decreasing by the day.
Gunners - around 5,000 mercs and decreasing by the day.
Mirelurks - around 10,000 and increasing by the day.
Goodneighbor - 500 citizens and 100 guards and decreasing by the day.
Diamond City - 1,000 citizens and 150 guards and decreasing by the day.
Railroad - around 500 and decreasing by the day.
Deathclaw - around half

Do you have a source, or just guessing?

I'm pretty sure it's supposed to be humorous. To be sure, raiders have to be the largest segment of the population.

Still, the Brotherhood has a lot more vertibirds than that. You will see half that many get blown out of the sky on any given day ... usually shot down by three raiders with pipe guns. Really, you would think they would spring for a little bullet proof glass for the pilot or something.

It's a Bethesda game, you'll get nothing but a steady diet of Super Mutants, Raiders, and Ghouls