A small featured image is not centered in Twenty Twenty theme

[pierlon]

Bug Description

When a small featured image (with a size below the recommended 1200x675px) is used for a post with the Twenty Twenty theme activated, the image is not centered on the AMP page.

Closely related: #4335.

Expected Behaviour

The featured image should be centered on the AMP page if a small featured image is used.

Steps to reproduce

1. Activate Twenty Twenty theme
2. Set a small featured image on a post, for example:
   
3. Verify that the featured image is not centered on the AMP page

Screenshots

Additional context

• WordPress version:
• Plugin version:
• Gutenberg plugin version (if applicable):
• AMP plugin template mode:
• PHP version:
• OS:
• Browser: [e.g. chrome, safari]
• Device: [e.g. iPhone6]

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