I'm sure plenty of people here can attest to the cascade of overhiring that happens to startups all the time.
For example, when Instagram was purchased by Facebook they only had 13 employees and now they have something like 500.
- manages get promoted
- CEOs justify increasing their salaries and bonuses
- allows taking away valuable employees from [potential] competitors
- when the headcount is big enough, companies acquire serious political weight
- often a branch or even an entire company can be used as training/proving ground to select the best employees to work on more valuable projects
The sweet spot for a startup is if you can look at that big mess of features and find something actually do-able by a small team that can still meet the needs of some portion of your target market.
When you've achieved that, you can then start filling in the rest of the features that will fill the needs of the rest of your market. This is where the headcount can grow significantly, because even small line items in the feature set could involve a big project, yet still be justified because you have enough potential or actual revenue to justify the team growth. Once you're in this phase, the communication overhead of adding people and teams also forces the creation of meta-teams to support the rest of the teams...and there you have it, you now have a lot of people.
In most markets, the average customer is a minority of customers. There is a lot of variety in each niche's demand. Maturity is having market coverage of those niches. Whether that coverage needs to be done by a single company is another question.
They are not all engineers. I’ll bet most are account managers looking after big advertising spenders.
Anyways, I'd agree that gainful is ideal, but I think most humans prefer to be employed and occupied over having nothing to do.
I wouldn't necessarily pull out "armchair experts" type comments for someone expressing curiosity / wondering what they're all up to.