This is the Beta 10 release of Phaser 3.
The phaser.js is a dev build, ideal for local testing and includes full source map data and comments (hence the huge file size).
phaser.min.js is a minified build, with no source map and comments stripped. You can use this if you'd like to upload a test somewhere and save some bandwidth. For development though we strongly recommend the source-map build instead.
Check the Dev Logs to see what is contained in this new build.