Reducers are used to update the state for a webview. They take in a state parameter, update it with the new state changes, and return it at the end. The webview controller base class automatically sets this.state = state at the end of the reducer's execution.
If you want to update the state (and force a re-render) mid-reducer, you can manually call this.state = state within your reducer definition. Do not update properties of this.state directly (e.g. this.state.someProperty = "new value") because then the actual state is out of sync with the local state variable that was passed as a parameter.
When you build the extension, webviews-metafile.json will be generated at your repository root; build using yarn build --prod to get a version without all the debugging sourcemaps. Upload this to the ESBuild Bundle Analyzer to get a visual breakdown of what's taking up space.