Alrighty. This isn't GIMP-specific, but it is an image issue.
I was making a wallpaper from today's APoD. The image is at:
Anyway, the filesize is 184 KB, and in JPEG.
So I used the GIMP to scale it back to 1280x1024, and I resaved it as a PNG. And its new filesize is 1.3 MB. Which is huge.
I'm curious if anyone has any ideas as to why this is, or ways to fix it.
It is because .png is not equal to .jpg. One of the best formats for compression vs quality is .jpg, however, I believe, and I have not tested this, .png compression is better at smaller file sizes. As well, .jpg does not support translucency or transparency as .png does.