GUIX pixelmaps dimensions

I reduced the size of a pixelmap on GUIX from 24KB to 6KB, but when I add it to GUIX pixelmaps, it shows always 24KB as size.

Then when I build the project in e2studio, the text value shown at the end of operation has been reduced only from 4202640 to 4202224. Is it normal?

I expected to recover more space reducing the size of the pixelmaps.