Comment by tdeck
I agree with you that they're not small in absolute terms, but I think these offices wouldn't make the mental list of top 4 tech employers in the Bay Area, and there's a very steep drop off after that. Maybe it would be better to say it feels like there are 4 big employers and almost nothing after that. All of this is hard to research - I tried doing some googling for numbers to validate my gut feeling but I wasn't sure how reliable they are.
Seattle metro is also a lot smaller than Bay Area metro -- and the Bay is probably the densest of all tech areas.
That said, Seattle does also have a good collection of "minor" companies, such as PACCAR, Expedia, Zillow, Tableau, F5. And I think Apple also has an office in Seattle (although probably really small).
Unless you want to work for one of the "brands", there are some good choices in Seattle.