EnTT is a header-only, tiny and easy to use library for game programming and much more written in modern C++.
Among others, it's used in Minecraft by Mojang, the ArcGIS Runtime SDKs by Esri and the amazing Ragdoll.
To be comparable in size we should look at what they do. The three game engines originally mentioned have everything from physics to an entire visual IDE. A comparable smaller project is Godot not entt.
A comparable ECS implementation is what you’re after. Is entt tiny in comparison to similar projects.