there're several different methods of doing this which are for example :

1. lossless compression(for PNGs)
2. lossy compression(for JPGs)
3. convert to grayscale(for JPGs and PNGs)
4. lower the color depth(for PNGs. from 24bit to 8 or less)
5. lower the resolution(for any. to 150 dpi or less)
6. resize the image(for any. from 3000x1500 to 1000x500)
7. leave them alone(for quality prick!)

Please share your thought
I don't compress them. If the cover image is too small, I'll replace it with a large one.
Once you remove something, it is near impossible to put it back the way it was.
At best, programs that enlarge, guess how to do a gradient between availble areas. (some are pretty good).
Seeing other of your posts; you really need to upgrade or add additional LOCAL storage.
Drive clone kits are fairly inexpensive. I have a couple of 2002 Laptops that had 80G drives, that now have 500G SSD for under $100 US (The kit just makes it handy, I previously just used longer cables in an old desktop (cloner system does not need to be target pc).

Clonezilla even did dual boot drives the the software that retail drives came with messed up.
it's not just about the price but how efficiently the storage is being used. 50% average compression means you can double your collection with same disk space occupied. I may sound paranoid but for the most part the book images aren't something you wish to keep for a lifetime but more like single-use and leave.
You will seriously find both sides of that fence around MD members.
There is the Higher Res cover crowd (and a plugin). And there is those on your side.
And I am a book hoarder. I have boxes of paperbacks (900+) I bought over the years. OTOH I standardized on a ~450px wide cover (I use resize cover PI to SHRINK the larger ones) in Calibre.
YMMV so have at it.
That is the beauty of Calibre. Your Library, Your way