Problem loading asset bundles with our own caching mechanism

I'm trying to use asset bundles with my own caching and loading. The process of downloading and writing works fine:

 using (var www = new WWW(url)){
 ....
         File.WriteAllBytes(bundlesFolder + bundleName, www.bytes);
  }

The issue is with the loading. I've tried using File class:

 byte[] bytes = File.ReadAllBytes(bundlesFolder + localBundlePath);
 AssetBundle bundle = AssetBundle.CreateFromMemoryImmediate(bytes);

I've tried "using www" encapsulating:

 AssetBundle bundle = null;
 using (WWW www = new WWW("file://" + bundlesFolder + localBundlePath))
 {
         yield return www;
         if (!string.IsNullOrEmpty((www.error)))
         {
               log.ErrorFormat("WWW Error: {0}", www.error);
          }
         else
          {
               bundle = www.assetBundle;
           }
  }

The problem is that on large asset bundles (>10MB), the code above causes iOS to crash due to high memory usage. Loading the exact same file using WWW.LoadFromCacheOrDownload on the same device does not crash.

 using (WWW www = WWW.LoadFromCacheOrDownload ("file://" + bundlesFolder + localBundlePath))
 ....

What am I doing wrong? Are there other ways to load the bundles from a local file?

(AssetBundle.CreateFromMemory doesn't help either. And AssetBundle.CreateFromFile isn't an option since our bundles are compressed (we use the deterministic option))