next.js/examples/with-webpack-bundle-size-analyzer/next.config.js

const { WebpackBundleSizeAnalyzerPlugin } = require('webpack-bundle-size-analyzer')
const { ANALYZE } = process.env

module.exports = {
  webpack: function (config) {
    if (ANALYZE) {
      config.plugins.push(
        new WebpackBundleSizeAnalyzerPlugin('stats.txt')
      )
    }

    return config
  }
}