What does “The code generator has deoptimised the styling of [some file] as it exceeds the max of “100KB”” mean?

This is related to compact option of Babel compiler, which commands to “not include superfluous whitespace characters and line terminators. When set to ‘auto’ compact is set to true on input sizes of >100KB.” By default its value is “auto”, so that is probably the reason you are getting the warning message. See Babel documentation.

You can change this option from Webpack using a query parameter. For example:

loaders: [
    { test: /\.js$/, loader: 'babel', query: {compact: false} }
]