I am into taking large volume enemas, and regularly take six quarts. Every now and then, I've taken eight quarts, but that is a sometimes thing and most times not. I am incredulous about 10 quarts or 12 quarts.
Lets see, a quart of water weighs about 2 pounds. 4 quarts weighs about 8 pounds. 6 quarts about 12 pounds. 8 quarts about 16 pounds. Large volumes do stress internal muscles. I've felt the weight.
As far as the colon goes, it's mostly stretchy protein. My understanding is that if you remove an animal colon from the body you can blow it up like a baloon. So the colon is not the limiting factor. It's the container (i.e. abdomen) it's in that is the limiting factor.
The abdominal muscles are a fixed size and shape. I've been fat & I've been thin, and in general, thin is better, since the abdominal muscles are also mostly protein and can stretch. Fat does not stretch. A layer of fat acts as a girdle that does not stretch. Fat also interlaces the abdominal muscles and makes them less stretchy. (Think of marbled beef: that is fat interlaced in the muscle.) The problem here is at what point do you cause a rupture in the abdominal wall. There was a male poster on another thread who claimed he got a hernia bubble while giving himself a five quart enema and it was very expensive surgery to correct. (I've been more careful since he said that.) I have a friend who, for other (non-enema) reasons, had surgery on her abdominal muscles and the doctors overlaid nylon mesh to hold the abdominal muscles together. The healing muscles grew into the mesh. It took her months and months of special care to heal. So obviously, rupturing the muscle wall would be bad news.
In general, a larger person is a bigger container and can hold more water. Women have wider hips which may allow them to contain more water, but they are usually smaller persons. Warm water helps too since the protein in muscles will stretch more when it is warm. Gas in your colon is a real limitation since it causes uncomfortable bubbles in the colon.
I am six feet tall and 245 pounds and again regularly do six quart enemas. Even then, I usually start with one or two four quart clean outs. I too would like to know how to get the ileo valve to open (on demand.) I've had it happen and felt the water flowing into my small intestines and been able to take more water in an enema. I usually take my enemas fast, but have found that I can hold more water if I take the enema slow. I think the water is being absorbed by my body like a sponge. Especially the liver.
Another limitation, if you are retaining your enemas for long, is water intoxication. Remember that woman a few years ago who drank two gallons of water to "Hold Your wee to win a Wii" and died of water intoxication. If you are getting headaches after your enemas, you are getting the effects of water intoxication. (It causes the brain to swell with water, and there is no room in the skull for the brain to swell.)
See this thread for the poster that got a hernia while taking a large volume enema:
http://www.zity.biz/index.php?mx=forum;ox=display;topic=72738