diff --git a/packages/next/src/build/utils.ts b/packages/next/src/build/utils.ts
index a47a8c83cfb8..83ae9c6f8e22 100644
--- a/packages/next/src/build/utils.ts
+++ b/packages/next/src/build/utils.ts
@@ -64,6 +64,7 @@ import { IncrementalCache } from '../server/lib/incremental-cache'
 import { patchFetch } from '../server/lib/patch-fetch'
 import { nodeFs } from '../server/lib/node-fs-methods'
 import * as ciEnvironment from '../telemetry/ci-info'
+import { normalizeAppPath } from '../shared/lib/router/utils/app-paths'
 
 export type ROUTER_TYPE = 'pages' | 'app'
 
@@ -110,14 +111,6 @@ function sum(a: ReadonlyArray<number>): number {
   return a.reduce((size, stat) => size + stat, 0)
 }
 
-function denormalizeAppPagePath(page: string): string {
-  // `/` is normalized to `/index` and `/index` is normalized to `/index/index`
-  if (page.endsWith('/index')) {
-    page = page.replace(/\/index$/, '')
-  }
-  return page + '/page'
-}
-
 type ComputeFilesGroup = {
   files: ReadonlyArray<string>
   size: {
@@ -773,6 +766,18 @@ export async function getJsPageSizeInKb(
     throw new Error('expected appBuildManifest with an "app" pageType')
   }
 
+  // Normalize appBuildManifest keys
+  if (routerType === 'app') {
+    pageManifest.pages = Object.entries(pageManifest.pages).reduce(
+      (acc: Record<string, string[]>, [key, value]) => {
+        const newKey = normalizeAppPath(key)
+        acc[newKey] = value as string[]
+        return acc
+      },
+      {}
+    )
+  }
+
   // If stats was not provided, then compute it again.
   const stats =
     cachedStats ??
@@ -788,10 +793,11 @@ export async function getJsPageSizeInKb(
     throw new Error('expected "app" manifest data with an "app" pageType')
   }
 
-  const pagePath =
-    routerType === 'pages'
-      ? denormalizePagePath(page)
-      : denormalizeAppPagePath(page)
+  // Denormalization is not needed for "app" dir, as we normalize the keys of appBuildManifest instead
+  let pagePath = page
+  if (routerType === 'pages') {
+    pagePath = denormalizePagePath(page)
+  }
 
   const fnFilterJs = (entry: string) => entry.endsWith('.js')
 
diff --git a/test/e2e/app-dir/app/index.test.ts b/test/e2e/app-dir/app/index.test.ts
index 271c9b654e72..cea3d44a7433 100644
--- a/test/e2e/app-dir/app/index.test.ts
+++ b/test/e2e/app-dir/app/index.test.ts
@@ -11,6 +11,12 @@ createNextDescribe(
   },
   ({ next, isNextDev: isDev, isNextStart, isNextDeploy }) => {
     if (isNextStart) {
+      it('should have correct size in build output', async () => {
+        expect(next.cliOutput).toMatch(
+          /\/dashboard\/another.*? [^0]{1,} [\w]{1,}B/
+        )
+      })
+
       it('should have correct preferredRegion values in manifest', async () => {
         const middlewareManifest = JSON.parse(
           await next.readFile('.next/server/middleware-manifest.json')