The costs to hire management and "support staff" like TPMs that scale with SWEs that help them meet goals is proportional to SWEs - often that is taken for the higher end fully loaded costs, depending on how you define it. Office space in downtown SF, Mountain View, or Palo Alto costs more than office space for back office workers in Nashville or Utah. Firms that hire SWEs often have fringe benefits like free food etc. and while they may apply to all workers, it tends to go along with hiring lots of SWEs.

But yeah, double is insane. When I saw prices for COBRA from Facebook, it was $3300 a month, and that was god-tier insurance - the insurance benefits were so good they had a custom list of what was covered that was probably way better than anything available on the market (e.g. you want brand name drugs? no problem. You don't want to try both ambien and trazadone before taking a sleep medication doctors actually recommend? No problem - etc.) - but for my needs it was barely better than COBRA costing way less than half. $3300/mo, or even $1200/mo for an entry level ops worker is a lot of their salary, and probably where the double comes from. At SWE compensation most of it ceases to scale.

The fully loaded costs including proportional management costs isn't relevant to the true marginal engineer, but estimates I've gotten from higher-ups definitely factor into engineering decisions about "should we spend engineering time to save money/make more money - how much will doing this thing cost the company" (opportunity costs are also relevant, but usually less grounded, since most projects don't have concrete benefits like "we will save $x/yr in infra costs")