I'd like to confirm that you have a larger size of the thumbnail available for Hub to access. Unfortunately, I cannot do this for you because it requires an authentication token, but if you could kindly verify it for me:
1. Go to https://cityofsydney.maps.arcgis.com/home/item.html?id=295675f834174b08bb8d4ea2af5e0d42
2. Right-click your thumbnail image and select "open image in new tab"
3. In that new tab, go up to the browser URL bar and at the very end of everything typed in there (it might be really long with your access token), add &w=400 to the end. No spaces. Just insert your cursor at the end of the URL, type in &w=400 and press enter.
If you have a larger image associated with the Item, it will be displayed, and if so, please let me know (or you can take a screenshot and post it here.) If you still see a small thumbnail (200x133), then I do recommend you follow Amanda Bishop's suggestion and re-upload a larger image because for whatever reason the larger version hasn't been attached correctly.