diff --git a/src/packager/large-assets.js b/src/packager/large-assets.js
index 60dc6bc..061038b 100644
--- a/src/packager/large-assets.js
+++ b/src/packager/large-assets.js
@@ -20,10 +20,12 @@ module.exports = {
   },
   scaffolding: {
     src: 'scaffolding.js',
+    estimatedSize: 4775503,
     type: 'text'
   },
   addons: {
     src: 'addons.js',
+    estimatedSize: 14339,
     type: 'text'
   }
 };
diff --git a/src/packager/lib/xhr.js b/src/packager/lib/xhr.js
index 81e1580..918029c 100644
--- a/src/packager/lib/xhr.js
+++ b/src/packager/lib/xhr.js
@@ -1,8 +1,11 @@
+const clampProgress = (n) => Math.max(0, Math.min(1, n));
+
 const xhr = ({
   url,
   type,
   progressCallback,
-  timeout
+  timeout,
+  estimatedSize
 }) => new Promise((resolve, reject) => {
   const xhr = new XMLHttpRequest();
   xhr.onload = () => {
@@ -19,6 +22,8 @@ const xhr = ({
     xhr.onprogress = (e) => {
       if (e.lengthComputable) {
         progressCallback(e.loaded / e.total);
+      } else if (estimatedSize) {
+        progressCallback(clampProgress(e.loaded / estimatedSize));
       }
     };
   }
diff --git a/src/packager/packager.js b/src/packager/packager.js
index 5034fb7..763c096 100644
--- a/src/packager/packager.js
+++ b/src/packager/packager.js
@@ -83,6 +83,7 @@ class Packager extends EventTarget {
       result = await xhr({
         url: asset.src,
         type: asset.type,
+        estimatedSize: asset.estimatedSize,
         progressCallback: (progress) => {
           dispatchProgress(progress);
         }