The previews are just not intended to be “temporary” so far. They will be created on demand for every image file in different dimensions (depending on requested for file list, details, gallery app etc) and will stay there until the image file is removed. All previews for one image can have around the same size than the image itself, depending on the original image size of course. So the folder cannot grow “unlimited” and you have a rough idea what the maximum size (all previews for all images are created) will be.
Discussions around that preview function so far went into the opposite direction. People found the preview generation on files app and especially gallery app too slow, as long as there are not yet previews generated. If you upload a new gallery from holiday and want to share it e.g. to friends/family, they will face a relatively slow online experience, until all previews were generated. Therefore the preview generator app was release, so all previews for all images can be pregenerated e.g. over night or before you share the image folder to others. So I didn’t yet found the idea to have those previews removed (for still existing images) after certain time .
If you don’t need the whole preview thing, you can deactivate the feature through your config.php as mentioned, but of course also the previews on demand will be not generated anymore.
An interesting feature would be for fast hardware to just create the previews on demand, but not save them on the harddrive.
As solution for you I could imagine a cron job, that daily scans the previews folder for subfolders (related to single files) that were created > 30 days ago or what. Should be not too difficult.