Fist of all you have to adjust what kind of previews you would like to have. I have had the same problem and have to setup my config.php as following to have only FullHD previews maximal (default is 4096):
'enable_previews' => true,
'enabledPreviewProviders' =>
array (
0 => 'OC\\Preview\\PNG',
1 => 'OC\\Preview\\JPEG',
2 => 'OC\\Preview\\GIF',
3 => 'OC\\Preview\\BMP',
4 => 'OC\\Preview\\XBitmap',
5 => 'OC\\Preview\\Movie',
6 => 'OC\\Preview\\PDF',
7 => 'OC\\Preview\\MP3',
8 => 'OC\\Preview\\TXT',
9 => 'OC\\Preview\\MarkDown',
),
'preview_max_x' => 1080,
'preview_max_y' => 1920,
More about previews config is here: https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/config_sample_php_parameters.html#previews
After you change those settings you need to delete all files in preview folder and perform some DB cleanup and generate new previews. Here I open a ticket regarding documentation and solutions are listed there:
I wrote small script to put previews generation status into my NC Logs: How to know if Preview Generator is working?