At nominal volumes, there can be a difference depending upon the enema solution. Obviously, someone who tries to take on a 5 gallon enema is going to find it both hard to hold and painful.
For me, the confluence of hard to hold and painful is a very soapy and very high, variable concentration glycerin enema. That is a 15%, 4-liter soapy enema. But it's put together in a particular way. It's 600 ml of glycerin added to bag and then allowed to fill the tube (and in the case where I'm using my tube with the Higginson pump, filling the pump as well). This is followed with the addition of 3.4 liters of warm water that has a full 90 gram bar of Ivory soap (shaved/grated) dissolved into it. I carefully pour the soapy water into the bag. Although there is some degree of mixing between the glycerin in the bag and the soapy water, there is still a degree of stratification in the bag (it is visible in my clear bag) and the first 150 ml out the tube IS pure glycerin.
After inserting and inflating the double balloon nozzle, when I open the flow control valve, the flow through the nozzle is quite slow (glycerin is pretty viscous) and probably coats the upper rectum/lower sigmoid. As the first 200-250 ml flow in, the concentration of glycerin begins to drop from 100%, but I have no easy way to determine what the concentration is. By the time the first liter is in, the visible stratification between the glycerin layer and soapy solution layer is gone. Basically, from there to the final amount I let flow in, any mixing is occurring in my colon.
At these strengths, the ability to hold it any length of time is challenging and it takes a fairly concentrated effort on my part to hold it beyond 10 minutes. But it can also be painful because the colon seems sensitized by the high glycerin concentration while the strength of the soap tends to feel more like a burn. I've done each of these enemas individually (glycerin only and soap only) and I respond differently to the individual component solution compared to the combined.
The "cramps" can be painful because rather than just strong muscle contractions, they act more like a muscle cramps.
The glycerin is a very sensitive tingle at these levels.
The soap is more of a burning sensation.
It is an interesting combination of pain and pleasure. And only once did I think I shouldn't have done this type of enema.