Every image (or set of images) downloaded from a Sprout Studio gallery has a download preset applied. A download preset is a combination of a size (web, medium, large, high-res, and original-res) and an optional watermark per permission level specified.
You can specify the download preset for each permission level in a gallery in the gallery settings modal. Read more about enabling downloads in Sprout Studio galleries here.
When you or your client downloads an image or set of images from the gallery, they are optimized and resized to the resolution that you specified in the download preset. This is similar to resizing an image in Photoshop.
That's why the downloaded image is a smaller size than the image you uploaded.
To deliver original resolution files, without any resizing or optimizing, choose "original resolution" for your download preset.
Read more about setting up your download presets here.