Guess I am in the 5% group that regularly uses six quart enemas. I took a long time developing that too. Through my 40's and early 50's I could do four quarts max. But I have always pushed for more volume. Sometime at the end of my 50's I became able to do six quart enemas and have done them ever since. On occasion, I have pushed for more volume and a few times have done eight quart enemas. I can't do that with any reliability. And I don't recommend it. (I did it recently by taking two quarts and retaining it five minutes. Then another two quarts and retaining that five minutes. Then a third two quarts and retaining that five minutes. Then taking the final two quarts and believe me, you can not retain it long! I was also already "flushed out" by prior enemas. I am also rather certain the valve between my large & small intestines opened and water flushed into my small intestines. I also use warm water, 105° to 109°F. The warmth helps the muscles too stretch.
Usually, the first enema I take in series is only 4 to 5 quarts. An then I expel. The second enema may be 5 to 6 quarts and then I expel. I use warm water too, since that helps the muscles stretch. I think the process of taking a large volume enema stretches the muscles at the time too, making higher volumes possible. After the second enema, I can usually take a series of six quart enemas without problems: flush in and flush out. It is very uncomfortable to hold a high volume enema for a period of time.
I haven't really planned or used 4 quart enemas for a decade or so. So yes, there may be a permanent stretching.
Also, for many years I uses an antique vaginal douche nozzle as my favorite nozzle. Around 2009 I bout a Klystra KP-2 nozzle. It helps me retain enemas. I've since tried to go back to my douche nozzle and it is so loos in my anus. Like a clapper in a bell! LOL!
Klystra KP-2 enema nozzle - It is not their biggest but it works for me.
http://www.klystra.com/store/enema-nozzles/tips/kp-2