“embed-thumbnails” folder in wp-content/uploads got huge over time

The folder mentioned is not from WordPress itself. It will probably come from a plugin or theme you are using.

I cannot currently find one that uses a directory with this name. So my guess would be that it is a commercial theme or plugin.

Alternatively, it could be from a plugin you used in the past that is no longer in your project. You should be able to recognize this by the last modification date of files in the directory.

Use the plugin “String locator” and search for the folder name string “embed-thumbnails” to find out which plugin generates those images.

Use an option in the detected plugin to disable those thumbnails, or configure it to reduce their filesize with smaller image dimensions and/or higher compression.
(Maybe it’s your cookie banner plugin, to show a thumbnail of an embedded Youtube video, before user consent is given.)

If there is no such option, maybe you need to replace the detected plugin (or book a hosting plan with larger webspace).

Thanks @cyrfer ! The String Locator was helpful. It’s the plugin “Real Cookie Banner Pro” – I would not have guessed that, but yes indeed, it does create and save preview images.

I’ll contact the creators to find out whether 3 GB is “normal”.