No idea, my guess would be the force touch gizmos, the trackpads tack on a fair bit of hardware[0] though I&#x27;ve no idea how much it weighs and in the &quot;disassembly&quot; pics the feedback thingy (&quot;taptic engine&quot;) seemed fairly large and mostly metallic: <a href="http:&#x2F;&#x2F;live.arstechnica.com&#x2F;give-us-a-hint-apples-september-9-event&#x2F;images&#x2F;DSC00147.JPG" rel="nofollow">http:&#x2F;&#x2F;live.arstechnica.com&#x2F;give-us-a-hint-apples-september-...</a> (that&#x27;s the big gray block in front of the battery, you can read &quot;taptic&quot; on the far side of the picture)<p>The different aluminium alloy might be a few % heavier, but it wouldn&#x27;t increase the whole phone&#x27;s weight by 10% (replacing aluminium by steel might)<p>[0] <a href="https:&#x2F;&#x2F;www.ifixit.com&#x2F;Teardown&#x2F;Retina+Macbook+2015+Teardown&#x2F;39841#s89855" rel="nofollow">https:&#x2F;&#x2F;www.ifixit.com&#x2F;Teardown&#x2F;Retina+Macbook+2015+Teardown...</a>