I am using version 2.0.1. The latest stable is 2.0.2. It won’t hurt to upgrade.
Size Issue
Adjust the quality of the output JPG. Your original is likely not 100% so saving it at 100% (G’MIC default) would make the file substantially larger. E.g., if I were to save it to 98%,
-o sample.jpg,98
it would roughly be 9x larger (4.20 MB). At 98%, there is no perceptual difference; in fact, GIMP confidently exports JPGs at 90% by default. Read the GIMP docs for more info.
In general, try different things like I have and read the docs or search this forum. Chances are the answers are relatively straightforward 
.