Featured Image size in post

Hi,

I need help with this, please.

I upload a picture to be Featured image at a certain size, but when I see it in the post, the image size has decreased.

For example, I upload an image in size of 1200 x 1034, but within the post it is displayed with the size 960 x 827. Even also changed the file name, for example I upload the image with the file name “myImage.jpg” but in the post the picture appears with the file name changed to “myImage-960×827.jpg

How I can do to make the system does not change the image size?

Thanks in advance !