This is an ongoing issue that we’re working on. In short, it’s because we need to switch to ES6 classes to make tree-shaking work, and we can’t switch to classes until we deprecate certain old files. This will happen in December, so hopefully the build size will go down early next year.
In the meantime, there are workarounds. My favorite is the one outlined by @cream. Note that it’s for Rollup rather than Webpack. But possibly it can be adapted to work with Webpack
If that doesn’t work for you, see my comment further which has a link to @drcmda’s manually trimmed three.js file. instead of importing from the main three.js file, you import from this edited file. However, you have to keep this file updated depending on what parts of the library you use so it’s extra work.