15 - [Inner] The Ward of Erahum [Mouse]

[Southern Crane]

PS: You will find that the 800mb file will be over 2GB once it's open in GIMP. I don't really know why that is, maybe due to compression. *shrugs* [Edit] Unless you mean the xcf-file was indeed 2.6 Gb even before opening in GIMP. In that case it sounds like you imported a gigantic layer (like 20kx20k) and didn't crop it to image/canvas size.

Yes its compression. When you work in PS or GIMP, its always uncompressed images you are working with. So if you open a naturally compressed format like JPEG its instantly going to get larger when you are working on it in Ps/gimp. When you resave in jpeg or PDF or what not it re-compresses the image. For work, I usually have files that are 40-50MB in Ps that, once saved in PDF format, are usually 4-10MB.

Just as an example, lets say you open a 10MB image in PS, then resize it in PS and then copy the layer into your project. You'll still have a 10MB addition to the project while its open in PS.
But if you resize the image smaller, save it in JPEG, close it, then re-open in PS then size of the new JPEG will be much smaller because you are getting the new uncompressed size.