Hi Patryk,
Thank you for such a fast response.
It helped a bit, now the images are scaled down from 448 to 353. However: why 448? Images on the home page are usually rendered as 353 pixels wide (for wide viewports) or smaller, but on the other hand, they can extend up to 512 pixels for narrow, single column view.
I still have some problem understanding why the featured image displayed on a single post view (the first image in each post) is taken from the full size and scaled down to 739 pixels instead of using the “large” sized version — even after setting the “large” size to exactly 739 pixels and recalculating.