Howdy!
A very large previews directory will almost always be due to transcoded videos, which are required for browsers to be able to render videos reliably.
PhotoStructure defaults to transcoding almost all video types to a standard MP4 format. If you’re only going to be viewing your library’s videos on, say, a windows or linux box whose browser has all the codecs necessary to render all your library’s videos, you can completely turn off transcoding by setting transcodeVideos to false. Settings documentation is here: PhotoStructure | Advanced Settings
After you change that setting, shut down PhotoStructure, delete the previews directory, restart PhotoStructure, and then run a rebuild via the main menu.
Note that most built-in smartphone browsers can only render a couple different video formats, so non-MP4 videos won’t be visible on your mobile after you make this change.
Also note: PhotoStructure has a bunch of related settings that let you control max bitrate of transcoded videos, and be more or less selective in what video formats get transcoded. Check out doNotTranscodeVideoCodecs and doNotTranscodeAudioCodecs, specifically.
Final note: know that PhotoStructure supports “hybrid” libraries. These let you store your previews on a fast SSD, and your original files on a slower and larger HDD. More details are here: Hybrid PhotoStructure libraries