Most organizations would have choked themselves on tens of thousands of bad hires long ago.
Combine that with how most the company spend all their time thinking about making money and that's probably why the company never succumbed to bloat
I also think they have sufficient career progression (in terms of problems solved and $$$ earned) that nobody feels the need to build a big team. Pure speculation though, I know nothing about RenTech except that the pay is... generous.
This is different from most orgs who can grow revenue through expansion of some sort, in which case the incentive often favors adding new employees. Not to mention the tendency for people in tech to be evaluated by how many people are in their org, further incentivizing adding headcount to signify your importance.
Obviously the number of people working on the super secret stuff is smaller, not to mentioned each project has a compartmentalized staff, but keeping secrets at this size is going to be a tough ask. They seem to do a pretty good job, but we know they’re not infallible at it. I imagine RenTech at 1/100th the size would have a vastly easier time.