A more general fix for all oneboxes has been created:
https://github.com/discourse/discourse/pull/22621
For testing I used articles from this non-wikimedia site: Read the latest Great Reads stories from car lovers like you - Hagerty Media as they had fairly large images to onebox.
Before:
After:
Before this change onebox images were around 400kb. After my change they are around 100kb.
Here is a onebox example which is using the reduced thumbnail size