Additonal thumbnail image gets generated although I have disabled it everywhere

Hello,

What I want to achieve – after image gets uploaded, original is kept and one specific image size is generated which is then used as a featured image.

Since I don’t need thumbnail generation, I went into settings->media and changed all values to 0, as well as unchecked every box (I read that it’s one way how to disable thumbnail generation)

Then I added this line to my functions.php
set_post_thumbnail_size( 1170, 660, array( 'center', 'center') );
in order to have that one image size that I need.

I checked whether there are any other values for add_image_size or set_post_thumbnail_size in my fucntions.php but didn’t find any.

At the moment I’m almost at the point where I want to be, but for some reason after uploading image an additional image gets created. The image is always 768px wide with proportional height.

At first I thought that this additional image size is generated by my template, but I tried unmodified Twenty Sixteen and Twenty Fifteen templates and the results were the same.