A nearly empty colon does provide more space for greater liquid volumes if you are into large numerical values.
However, I take enemas because I like the feeling of a nice (large) nozzle inserted and locked into my ass, the feeling of the warm enema solution of choice flowing into me and filling me, the feeling of being "full" and holding as long as I can (maybe to an assgasm or well beyond the first orgasm to the second cumming), and then the release in long and powerful contractions shooting out a stream of "spent" enema solution. I must admit that I do like the sensations of stool softened by the enema working its way down my descending colon, through my Sigmoid and out of my ass. Soap softened stool is particularly enjoyable because the muscle contractions from the soap are more intense as is the sensation of movement through the colon.
Besides, whether the volume is occupied by liquid or by "solids," volume is volume. The difference is just a matter of localized density.
The only "problems" I ever had with a full (or nearly so) colon is that liquid flow may have to be slowed appreciably to allow the liquid to find its way around and through the contents of my colon. That isn't really a problem with a partially full to nearly "empty" colon. However, as I fill, the liquid finds channels to flow through and around the contents. The only other problem I've run into deals with either adequate sealing at the base of my rectum (through a large diameter nozzle WILL push the contents back up into my rectum) or threading a long colon tube into my colon around (or through) the contents of a full colon. Small diameter colon tubes could be stopped. Large diameter tubes (like my 1.6 inch diameter) has to push the contents back up the colon until it gets to the point where it can either push through or around the contents. My mid-sized long tubes work best in this situation as the 1-inch (2.54 cm) is able to through through even when my colon is the fullest. These conditions usually make for a very messy tube when extracted, but shit happens in these situations.
I must admit, though, it is a very pleasurable sensation to have such a volume of liquid on the "other side" of all the contents of a full colon to push out the contents from the far end of the colon.
YMMV!