The very first backup of a fixed size image will be the full size of the image, e.g. 50 GB, no matter which backup software you use. Even if inside the image no files exist. The very first backup of a sparse bundle disk image will be ~100 MB (the initial size of that image).
On repeated backups, some backup softwares operate on file level and upload the whole file if it changed. So if you have a fixed size 50 GB image, mount it, add a file, unmount it, it changed, and the whole 50 GB image file has to be uploaded (with some backup softwares).
