A couple of weeks ago I played (and finished) A Plague Tale, a game by Asobo Studio. I was really captivated by the game, not only by the beautiful graphics but also by the story and the locations in the game. I decided to investigate a bit about the game tech and I was surprised to see it was developed with a custom engine by a relatively small studio. I know there are some companies using custom engines but it's very difficult to find a detailed market study with that kind of information curated and updated. So this article.
Nowadays lots of companies choose engines like Unreal or Unity for their games (or that's what lot of people think) because developing a custom AAA-level engine requires lots of resources, so, I decided to list here some of the most popular custom engines with the team-sizes and notable titles released with those engines.
Most of the engines listed here have been developed along the years with multiple iterations and multiple videogames, those engines have gone through several versions or even complete (or semi-complete) rewrites from scratch, with a consequent engine name change. Also, important to note, most of those engines use numerous middleware for specific functionalities (Platform, Physics, Network, Vegetation, UI, Rendering, Audio...).
*NOTE: I tried to be as much accurate as possible with the information about the employees count (I checked the companies websites, Wikipedia or company LinkedIn) but take it with a grain of salt (some employees numbers could not be up to date).