Ash HN: Is there a concurrent database that fits between SQLite and PostgreSQL?

If you want a concurrent RDMS (Relational Database Management System) that is relatively light and easy to deploy, are there any contenders that meet this criteria?

Between the simplicity of SQLite and the complex, heavyweight that is PostgreSQL, it seems there is a chasm between these database opposites.

Is there a concurrent client/server database that fits in this gap while being simple, performant and easy to deploy?