In your Sprout Studio account, anything uploaded is counted towards your storage. This includes:
Design Proof uploads
The bulk of your storage allotment is usually in Galleries and Design Proofs. In most circumstances, images can be scaled and edited to make them easier to post with no impact on the image's quality.
This can be accomplished with JPEGmini, a program that reduces the file size of JPEG images by a factor of two to three, occasionally more, with minimum or unnoticeable image quality loss. You can also do this with Lightroom or Photoshop by exporting at 90% quality instead of 100%. The difference between the 2 files isn't noticeable without zooming in to the image.