BABEL Note: The code generator has deoptimised the styling of "app.js" as it exceeds the max of "100KB in Meteor

This is not a real issue. Just a warning. When babel compiles some code, it try to offer a readable output, but when files become big (>100KB), babel considers (by default) that it's not useful to keep this option enabled (because yes it's an option, see https://stackoverflow.com/a/30879872/988941 for more information).

It's not a problem to keep it. If you don't care about readability, you can just use compact: true. Or false if you do. Only "auto" value (default value) will print this warning.

At first for more explanation read the Babel documentation on Options.

It is a common option of Babel compiler that commands not include superfluous whitespace characters and line terminators. sometimes ago its threshold was 100KB but now is 500KB.

And I advise to you disable this option in your development environment, with this code in .babelrc file.

{
    "env": {
      "development" : {
        "compact": false
      }
    }
}

For production environment Babel use the default config which is auto.