While an interesting observation, and good to be backed up with fact/analysis, it's not particularly surprising - as companies grow and mature, their character changes and attract a different sort of employee. Gone, perhaps, are the younger, more risk-taking employees and in come the older, more risk-averse employees.

I doubt that "start-up culture" is what is required - I'm sure that the variety of work that comes with a smaller number of employees is a huge factor. Maybe they employ too many workers, or perhaps there isn't the ability to cross teams/functions.

What would be more interesting would be if those "tech giants" dropped in average talent as a result (though good luck in measuring that!) or even in total employee terms.