There are couple of other things which you can do to improve the build size,
- You can upgrade to webpack2 which includes a feature called tree shaking enabling it to include only required functions from an imported module. Upgrading from webpack 1 to 2 is straightforward, learn more about it here.
- As @sunnysingh mentioned, you can do code-splitting with react-router which loads only the required modules for that page. You can find how to do it here
Also, have you removed the devtool option in production webpack config?