featured image size and exceprt

I am running Nirvana theme on a private website with post slider on the home page

In the beginning I set the size of images for the slider to 1920×580 and uploaded as featured of the posts images cropped “ad hoc” with such aspect ration. The images were actually larger but mantaining the same aspect ratio I relied on resizing feature and everything worked.
In particular the pages with list of all post of a specific category showed the featured image and, on a new line, the exceprt of the post.

Then I realized that 1920×580 aspect ratio was somehow too thin for the images and changed it to 1920×870.
I changes the settings of the slider and started uploading as featured image of the posts images with this new aspect ratio.

The weird thing is that with these images the page listing all posts of a same category now behave in a different way with the image not spanning to the full width of the page and the exceprt beginning inline with the image with few words per line and not beginning on a new line.

I also tried to rebuild all the thumbnails (hint from a previous post in Nirvana support forum) but the behaviour is still the same.

Is there any setting that I should be aware of that I did not change?

Note for the author:
the website is password protected.
If you could be so kind to write me an email in private I will give you the password so that you can have a look to this Nirvana behaviour

thank you in advance.

paolo