You are probably doing this already, but in GIMP, go to "Image" > "Scale Image" and change the Width/Height units to "Percent" and change it to something smaller than 100% and hit "Scale". This should reduce the size of the image nicely.
For reference, though, when saving to an image format, remember that JPG will almost always be MUCH SMALLER than PNG (unfortunately), sometimes 2-3x smaller in size because it is a compressed, lossy format. PNG can also do compression, but mostly it doesn't compare in file size to JPG. Same with JPG being smaller than PDF (usually).
Forgive the noise if you've already tried this and knew that about image types, but that's what I would do.
- Abraham