In WatermelonDB's case, I bet most people will run it using React Native (ie mobile apps for iOS and Android), so that wouldn't really be an issue.

On the other hand, sql.js (https://github.com/kripken/sql.js/) packs to about 2,6MB (see http://kripken.github.io/sql.js/js/worker.sql.js). I don't know about performance under heavy use of that. I don't know what SQLite backend WatermelonDB uses from ReactJS either.