I’ve now tried it myself and after some debugging found the issue: Scriptable seems to scale the original image down to 500 px in the widget probably because of memory limits. Since these limits don’t exist in the app, the image is loaded in full size.
Thinking about it, it doesn’t make much sense since to scale the image down, it needs to load the full image first. But maybe the image is loaded in a different thread that has more memory available than the widget and to use it in the widget it needs to be scaled down. I don’t know the reason, only @simonbs knows it since he made Scriptable.
To solve the issue you could instead of download an image for the whole week and crop it in Scriptable, do that step beforehand when creating the image and then download an image for each day.
If this is not possible for you, you could download the image on monday with a different script (or the same script, but only when it is running in the app), crop it there for each day and save them to your iCloud Drive/local file storage of Scriptable and use these files then in the widget.