Apart from Signal just generally doing a good job here, I see a few other possible factors:
* Signal doesn't see user content, so it doesn't have a content moderation team
* Signal is designed in such a way that it can't comply with most kinds of legal requests for user data, so it doesn't need a large team responding to those requests
* Signal gets some amount of pro-bono legal help, so it might not have as large an in-house legal team as other organizations
* Signal isn't trying to directly profit from user activity, so it doesn't need to study user activity or engagement metrics with a view towards profiting from them; similarly, it doesn't need to manage relationships with advertisers
* Similarly, it doesn't need to try hard to grow its user base (that would be desirable, but it doesn't necessarily increase revenue much)
* Similarly, it probably doesn't need to try hard to expand into other business areas
(I think these things are generally great. Yay Signal!)
It's also better from a managing perspective because if you have 10% of bloat and then have a bad quarter, you then have a lever you can pull to show you are getting stuff done. Its how managers smooth their performance, you overhire in good quarters to lower the profitability comps, and then fire to buffer up bad quarters.