Altho XMS Systems Image uploaded reduces your file sizes it is still a good idea to resize your images as close as possible to the size you are going to use on your website.
XMS Systems Image uploaded will resize your images to maximum 660pixels wide and reduce the image quality to 80% of the original. This usually results in a huge size difference while still having a good image quality. We settled on 660px because that would be the basis of the “responsive switch”. Anything above that is basically a waist and not necesary.
See this blog post titled “Optimal image size to use on my website” for why it is important to do this properly