I know everyone hates bringing back an old thread, but this one is still a top web search result for the issue of setting the image thumbnail file size limit, and I wanted to close the discussion with more details on what worked for me.
As others have said, the default maximum limit for which NextCloud appears to create a preview image is 50MB (size of the original image). You can test this by uploading various images in the 40-60MB range, and only the ones with a file size less than 50MB will have a preview image generated.
It was mentioned above to use a config.php setting to change this, but the specific parameter name wasn’t provided.
NextCloud’s documentation on preview images doesn’t have a parameter listed for this file size limit, but thankfully OwnCloud documentation does (where NextCloud was originally forked from) and appears to work in the current version of NextCloud.
Use the configuration line
'preview_max_filesize_image' => 100,
where the integer you specify is the file size limit in MB of the original images for which you want to stop generating preview images. It does appear you can set this to “-1” for unlimited.