Just because is stupid o'clock in the morning and I can't sleep, I conducted a little test...
I created an 8 gig unencrypted sparse image. Before I copied any files to it, it had 28.4 meg of "used" space. I copied 4.1 gig worth of files to the image and the used space increased to 4.89 gig or so. I deleted the files and emptied the trash and the used space went back to the original 28.4 meg. I also copied a single, large file (an Ubuntu VMWare fusion file) which was ~6 gig. As you can suspect, the used size increased to accommodate the larger file, but when deleted it returned to 28.4 meg of used space.
However, the sparse image file size on the Volume on which I created it always consumed the full, formatted size of ~7.95 gig regardless of whether it had files or it was empty.