Image is physically resized

Site URL: https://www.thezimaconfession.com/

I uploaded a panorama image here -  https://www.thezimaconfession.com/other-photos (Paris from Montmartre). This image is  1378x800 - which is big. I wanted people to be able to right-click and see the image in a new tab - full size. After I uploaded it I tried right-clicking and the photo was much smaller and crappier than I expected. I downloaded it to find out what had happened and it was 750x345 - so it has been physically altered.

Why is this? It's especially strange because I recently uploaded some huge images by accident that were not resized (but they were not panoramas). Those photos were approx. 3Mb - whereas this one would be a mere 900Kb.

I've attached the image I want to upload to my site.