If you are using webpack in Rails via Webpacker, then the webpack configuration is not entirely straigtforward. Below is how I setup Webpack Bundle Analyzer in my projects.
First, install the dependency
yarn add -D webpack-bundle-analyzer.
Then add the following to yout webpack developmen configuration:
const BundleAnalyzerPlugin = require('webpack-bundle-analyzer').BundleAnalyzerPlugin; environment.plugins.prepend("BundleAnalyzerPlugin", new BundleAnalyzerPlugin());
You will now get a nice visual overview of your bundles when you run