This little procedure just saved me over 20 gigs (yes, this is right, gigs!!) of space on my hard drive. It has to do with how Lightroom updates. Read on or watch the video.
Here is something I was quite surprised to realize. With each catalog, you can set how long it would keep 1:1 previews. I usually set this for a month.
Keeping 1:1 previews is quite useful as it really speeds up using the develop module. Sadly, 1:1 previews do take quite a bit of space. This is why Lightroom has a feature that allows you to automatically delete those previews after a while. For me this duration is set for one month. After one month Lightroom will automatically delete those previews. This is a great balance between usability and space.
So each time that you run Lightroom, it looks at the catalog and checks if it needs to delete some old previews. But when you update a major Lightroom version, you are also updating the catalog (including its directory). So that old catalog will never run again, and Lightroom will never delete all those old previews.
I was quite shocked to realize that my two-version-ago Lightroom installation had over 10 gigs of previews.
Those were obviously deleted. There you go, I hope this saves you quite a bit of space as it did for me.