[xubuntu] why are GIMP saves are too big

My hobby is photo manipulations, and I use GIMP, but one thing that always bothered me was that GIMP saves are big, but I managed to workaround it using IrfanView to re-save the image, for example, when I created a 1920x1080 jpg in GIMP, the size of the saved jpg is about 2MB, but when I re-saved it using IrfanView, the image's size was cut down to 350KB, and I compared the quality of the two jpgs, same resolutions, no blurring or pixelation problems, no difference in quality between the two.

Is there anyway to fix this? Because using IrfanView to re-save my work is kind of a hassle.

I'm ruing Xubuntu 14.04 and I'm ruing Wine 1.8.2.