I have a real head head-scratcher. I am trying to boost the performance of my wife's website. One of the things I identified as potential issue was the size of image files. I am somewhat aware photo editor so I always tried to use the quality slider in GIMP to keep the file size reasonable. I've started to play with quality settings further in order to squeeze more performance but when I looked at my images via filezilla I was shocked, screenshot attached. As I understand, Wordpress automatically makes multiple resolutions of uploaded images in order to serve them to differently sized screens and thus save bandwidth but look at the file sizes I got!

in most cases image one step smaller in resolution is twice as large in disc space a in some cases more than three. I tried to find what compression settings WP uses and it seems that after version 4.5 it should be 82% which doesn't seem to be the case for me. I am certain that I did not tamper with this parameter by any custom filter but I cannot be sure if about the theme or plugins that I use.

here is the info I have on media handling (headers roughly translated, my WP dashboard is in Polish):

Active Editor : WP_Image_Editor_Imagick
Version of ImageMagick : 1690
ImageMagick version designation : ImageMagick 6.9.10-57 Q16 amd64 2019-09-05 
Imagick resource limits : 
    area: 384 GB
    disk: 9.2233720368548E+18
    file: 7500
    map: 384 GB
    memory: 192 GB
    thread: 1
Version of GD : 2.2.5
Version of Ghostscript : Could not verify if Ghostscript was installed

I would like to ask 2 questions.

1. Is there a way to find out what are the current compression settings used by my WordPress ?
2. I think I can write a script that will take all those automatically generated files in the uploads. directory and re-compress them to my desired size. I do not know if WP will not freak out that the files were changed externally and thus my question : would such approach be even advisable ?

WP version that I use is: version: 5.2.7