As far as I know, the Outlook Classic you downloaded should be the same as the one that you had on your Win10 box.

My first guess would be that there is something different about your sync settings. I sounds like you used to do a regular background sync (so that when you opened a large item is was already downloaded).

Configure Gmail Accounts in Outlook

Change send/receive group settings - Microsoft Support

But you say it's only on one item and that doesn't really fit my guess. Anything unusual about the newsletter? Is it larger than your normal mail?