How are you manipulating the image size chosen for that page? Within the dashboard of WordPress on that page or within the theme files?
I think what might be happening is that you’re manipulating the smaller image size rather than pulling the larger image size itself.
While into the Page editor,
http://spesagodina.it/web/wp-admin/post.php?post=4525&action=edit
I click on the image and I edit the image trough the wordpress image editor (image details) and there I change the views settings.
Do you have a child theme at all? If you’re not familiar with manipulating the theme php files, I would suggest uploading a larger image size and making it smaller rather than vice versa.
Regenerate Thumbnails will continue formulating the same image sizes without editing those files.
I am not using child theme. the image is large a bit more of 2000 pixel, thanks to the plugin Imsanity https://wordpress.org/plugins/imsanity/ .
And the original image is 4000×2248 pixel so really larger of a thumbnail of 55 pixel width…
At the end I skipped the problem changing the size of the medium image to 55 max width instead to use the custom thumbnail size.
