I was giving myself two quart enemas as young as twelve, possibly eleven (memory fads, alas.) This volume continued until I was in my thirties. Initially I inadvertently tried to take more volume when I obtained a hospital barium enema kit, which held much more liquid than the bag markings indicated. I was about 37 when I attempted a "real" three quart enema. I managed to finish that enema but it was a challenge.
Over the following decades I increased my capacity to four quarts, typically taking three and a half quarts with no problems.
More than any other factor I think my capacity increased due to practice. Much like when I first tried to use a colon tube. The first time I tried it I was not able to insert more than half its length so I gave up. I revisited colon tubes years later and with practice learned to insert the entire 30". That skill has not lessened.
I stopped taking larger enema several years ago, around the time that my colon began rebelling against coffee enemas. Now, at 67, two and a half quarts is a filling enema.
Undoubtedly, like any other practice, straining the colon's capacity will lead to larger volumes, but at least in my case, it isn't permanent.
PS
I like your avatar image.