The amount of water from a colema is much higher than from an enema. The amount of water used in a colonic is much higher than that. Basically, the sheer volume of water being used helps get more water further up into the bowels and also helps clean up and push out more 'stuff' in the bowels.
Think of washing at the sink (enema,) or under a water hose (colema,) or in a full jet shower (colonic.)
However, before thinking "I'll just do a WHOLE LOT more enemas" know that there is such a thing as too much of a good thing. Too many enemas in a day can be very dangerous. If thinking of doing more than 2 enema bags or one colema bucket, it would be far safer to see a professional and have a colonic done instead.