Featured image masonry mode

Hello,

I have a very specific problem i was unable to solve for months and tried to ignore.

When the “blog” page is in masonry mode it uses 450×222 pixel sized featured images and WP crops them automatically to that size.

From the functions.php:
…
$img_width = $blog_layout_type != ‘masonry’ ? 900 : 450 ; //set width large img
$img_height = $blog_layout_type != ‘masonry’ ? 444 : 222 ; //set height large img
…

But when i open the post it displays the featured image in 870×429 pixel, but uses the cropped 450×222 sized image and gets very pixelated:
https://nebulan.hu/erdekessegek/mitol-kulonlegesek-a-pink-fury-folyadekok/

How can i solve this problem? ??