Image size is different in GIMP than desktop

[vakita]

I'm still a little confused but, if I understand correctly, if I want to continue to make changes to an image at a later time, I should save them as an xcf file. Also a jpg will loose some quality each time it is exported, so in order to avoid that I should use png. If I am finished making changes to an image that I want use on a website would you recommend that I export it as a jpg as opposed to png for the advantage of the much smaller size ?
I appreciate your taking the time to help. Thank you.