Twitter has to do everything itself - sales, marketing, customer support. This means that its vision is limited by what the executives can see, and they're going to look at traditional business factors for ideas. The political structure tends toward corporate fiefdoms over time.

Mastodon is leveraging forces from many different directions: first by building towards the existing GNUSocial protocol and its userbase, and then as different communities come in, getting donations of their time and skills to operate instances tailored to their needs. Businesses have angles on this too: for example Japanese art site pixiv made their own instance and client app. The federated model allows some divergence but also the "loose ties" of a common protocol. There is the prospect of embrace-extend-extinguish occurring, but this historically only happens some of the time, and it requires the full pressure of a giant tech firm and all their marketing strength to make it work.

Sales, support, marketing, accounting, business ops, software ops, management, HR. Pare those off and you might get something like Mastodon's contributor size after adjusting for scale of user base. Although I doubt many of Mastodon's contributors are full time, so a comparison is difficult.

So Twitter is big because it's big?

I don’t follow. Twitter is big because of all the things I just listed.

You just listed its components, the question was about why Twitter needs those things but Mastodon does not.

Because Twitter is a for-profit business, whereas Mastodon is not.

And they are up to around $2 billion in losses so far, right?

It sounds like they would be better off just being a Mastodon node.