Describe the bug
By default gutenberg tries to load the featured image with the image size post-thumbnail in the editor sidebar:

This image size does not exist by default. The correct image size would be thumbnail.
See the default image sizes in the Codex: https://codex.wordpress.org/Post_Thumbnails#Thumbnail_Sizes

Because the specified image size does not exist, the full size is used as a fallback.
This causes a performance issue, because a potentially far too large image is loaded.

To reproduce
Steps to reproduce the behavior:

1. Create a Post
2. Add a Featured image in a large size e.g. 2000px wide
3. Right-click on the image and choose "open image in new window"
4. You can see the image in full size.

Expected behavior
The Featured image should be loaded in a smaller version/thumnail site within the editor sidebar.

Desktop:

• OS: OS X 10.13.6
• Browser Chrome
• Version 74.0.3729.169

Additional context

• Gutenberg Version 5.8.0-rc.1