I am using LR Classic CC ver 7.4 My catalog is 5.37GB and when I try to backup and optimize it on shutdown, I get this message. If I use one of these apps, when in the process does it get applied? Do I just find the catalog with Finder and then compress it or is it already compressed and I'm using this app to unzip it? If so, when does that occur? I don't use zip apps so not familiar with the process. Also, pretty sure the catalog size is correct as I have a bunch of digitized 4x5 transparencies and over 4 years of digital photos on there. I read about creating separate catalogs by year, subject and stuff but then read that there shouldn't be any problem having a large master file. 
1 Correct answer
Lightroom compresses the catalog into a zip file when it creates the backup. The only time you will need to use the software to unzip the backup is if you need to resort to using the backup catalog. Remember that your images are not in the catalog. The catalog just contains the history of the work you have done as well as a link to where all the images are located on your computer.
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