Why Are File Sizes So Large For Upscaled Gigapixel Images?

I just upscaled a bunch of images and admittedly they were already large images so now they are 13000x19000 after upscaling. Each image is now approximately 1GB in size which is pretty ridiculous. I ran the images through XnConvert and it lowered the file sizes down to about 7MB each and it seems to have not lost any image quality (I’m viewing on an 8K TV). Why are the file sizes so huge when using Gigapixel when they don’t have to be.