Most REST APIs I've seen in the wild just send the entire object back by default because they have no idea which fields the client needs. Most decent REST API implementations do support a field selection syntax in the query string, but it's rare that they'll generate properly typed clients for it. And of course OpenAPI has no concept of field selection, so it won't help you there either.

With my single WordPress project I found that WP GraphQL ran circles around the built-in WP REST API because it didn't try to pull in the dozens of extra custom fields that I didn't need. Not like it's hard to outdo anything built-in to WP tho...