@BFusion: I've gotten so I regularly take six quart enemas. If I don't for a period of time, say three months, my first enema will be more like five quarts. But once I am cleaned out and using warm water I have no problems stretching back to six quarts.
Also, if you've gained or lost a lot of weight, this might affect your capacity. If you've recently been doing stomach crunches to get six pack abbs this might affect capacity.
If you have regularly been taking three quart enemas and suddenly, under similar conditions, find you can only take much less I would be suspicious that you have a medical problem. My suggestion: go see a doctor.
I've seen pictures on the internet of healthy colon x-rays and cancer colon x-rays. The healthy colon is of a similar width the whole length. But the cancerous colon is greatly narrowed in one section. This makes a blockage and would surely restrict capacity.
I am not a doctor or medical person, but if you have an unexplained reduction in capacity, I would be suspicious that something is medically wrong. Go see a doctor.
PS: I've used various dildos, of length & width, mop handles and even the handle of a baseball bat to get off. I've had them deep enough in me to touch my belly button from the inside. This has not changed my enema capacity at all. (I've done all that and still can't get a colon tube deeper than about 20 inches. So I don't suppose doing so permanently straightens out any curves.)