Lightroom Catalog Previews.lrdata 72Gb

@Ed_Nes 

You can delete the "Lightroom Catalog Previews.lrdata" folder entirely when LrC is closed. On starting LrC again, the "Lightroom Catalog Previews.lrdata" folder will be recreated and Previews will be rebuilt on an as required basis.

In the LrC Library Module, select the desired photos (or possibly all photos) and then select Library->Previews and from there you can delete whatever previews you want. Note: by deleting previews, the Library Module will run slower than before. This is a tradeoff, disk space is freed up at the cost of slower operation of the Library module.