Reduce previews.lrdata file size

All the images that you see in the Library module are previews of various sizes (for Navigator, Grid, Filmstrip, Loupe, 1:1, etc.). Those previews are stored in the Previews.lrdata package. You can delete it, but LrC will automatically recreate it and will start populating it with newly generated previews (which makes LrC slower). You could also try the Library menu>Previews>Discard 1:1 Previews, that might save you a small amount of space eventually (but every time you zoom into 1:1 in the Library module a new 1:1 preview is built again).

Depending on how you work, it might be practical to delete the Previews package, then when LrC recreates it you could just build Standard previews only for the last couple of years images. No point having previews for every image if you rarely access the older images.

The cache (I assume you are referring to the Camera Raw Cache) is unrelated to the Previews package (the cache is only used in the Develop module).