Make experimental stats file approximately 90% smaller (#21858)

Tested against some smaller apps. Will have a high impact for larger apps.

packages/next/build/webpack-config.ts

@@ -1113,7 +1113,8 @@ export default async function getBaseWebpackConfig(
           buildId,
           rewrites,
         }),
-      !isServer &&
+      !dev &&
+        !isServer &&
         config.experimental.stats &&
         new BuildStatsPlugin({
           distDir,

packages/next/build/webpack/plugins/build-stats-plugin.ts

@@ -8,6 +8,64 @@ import { tracer, traceAsyncFn } from '../../tracer'
 
 const STATS_VERSION = 0
 
+function reduceSize(stats: any) {
+  const modules = new Map()
+  stats.chunks = stats.chunks.map((chunk: any) => {
+    const reducedChunk: any = {
+      id: chunk.id,
+      files: chunk.files,
+      size: chunk.size,
+    }
+
+    for (const module of chunk.modules) {
+      if (!module.identifier) {
+        continue
+      }
+
+      const reducedModule: any = {
+        type: module.type,
+        moduleType: module.moduleType,
+        size: module.size,
+        identifier: module.identifier,
+      }
+
+      if (module.reasons) {
+        for (const reason of module.reasons) {
+          if (!reason.moduleIdentifier) {
+            continue
+          }
+
+          if (!reducedModule.reasons) {
+            reducedModule.reasons = []
+          }
+
+          reducedModule.reasons.push({
+            moduleIdentifier: reason.moduleIdentifier,
+          })
+        }
+      }
+      // Identifier is part of the Map
+      modules.set(module.id, reducedModule)
+
+      if (!reducedChunk.modules) {
+        reducedChunk.modules = []
+      }
+
+      reducedChunk.modules.push(module.id)
+    }
+
+    return reducedChunk
+  })
+
+  stats.modules = [...modules.entries()]
+
+  for (const entrypointName in stats.entrypoints) {
+    delete stats.entrypoints[entrypointName].assets
+  }
+
+  return stats
+}
+
 // This plugin creates a stats.json for a build when enabled
 export default class BuildStatsPlugin {
   private distDir: string
@@ -25,9 +83,21 @@ export default class BuildStatsPlugin {
             const writeStatsSpan = tracer.startSpan('NextJsBuildStats')
             await traceAsyncFn(writeStatsSpan, () => {
               return new Promise((resolve, reject) => {
-                const statsJson = stats.toJson({
-                  source: false,
-                })
+                const statsJson = reduceSize(
+                  stats.toJson({
+                    all: false,
+                    cached: true,
+                    reasons: true,
+                    entrypoints: true,
+                    chunks: true,
+                    errors: false,
+                    warnings: false,
+                    maxModules: Infinity,
+                    chunkModules: true,
+                    // @ts-ignore this option exists
+                    ids: true,
+                  })
+                )
                 const fileStream = fs.createWriteStream(
                   path.join(this.distDir, 'next-stats.json')
                 )