How do I analyze my webpack bundle contents

You can do one of the following:

use the built in webpack-bundle-analyzer by adding the analyze option to the build command:

yoshi build --analyze

upload the webpack-stats.prod.json file to a website:

Go to https://chrisbateman.github.io/webpack-visualizer/
Drag target/webpack-stats.prod.json from your project and drop in browser page for analysis.

use the CLI with the webpack-stats.prod.json: try https://github.com/robertknight/webpack-bundle-size-analyzer
Use the webpack bundle analyzer page:

Go to https://webpack.github.io/analyse/
Upload target/webpack-stats.prod.json from your project