Something i miss understand with media (image weight)

hi everybody

For the past few days, i tryed to improve speed of my website, so the first things i did, i compress all images to save KB, i re-upload them but when i check with the chrome console in source my image has weight bigger than the original uploaded :

myimageuploaded.jpg is 2.3 KB 461x461

but when i check source myimageuploaded.jpg located in the image folder is almost 10.4 KB how it possible ?
for the test i just insert image in the markdown editor the same happen when call it inside a twig, of course i clear cache.

I’m really confused because i don’t know if it something i done with config parameters or i misunderstand the way grav handle media.

So i will very be very grateful if someone can explain what’s going on ?