thats because the answer isn't always the same.
Sometimes You can completely saturate yourself w/ food and it will just sit around in your stomach untill all of it can be digested it could be there all day if you ate enough.
At the same time, sometimes the body does pass alot of it through and you find yourself in the bathroom a few ours later.
Now eating is one signal for the body to have bowel movements just becuase you ate alot and your in the bathroom later doesn't mean whats coming out is the stuff you just ate it could be comepletely different. Other times it it.
Generally this overflow can't really be used to your advatage nor is it something to worry about
IF you tryied eating all your food at once hoping to lose weight it wouldn't work, your body will become accustomed to it, and it will realize it has to digest it all. If your worried your eating too much and not getting the benefits of your protein ect.. don't same thing goes the body cranks itself up and becomes accustomed to your eating habits.
However there are times, (like my icecream example) when the body is just shocked, it can't handle what your doing to it. The result is your in the bathroom passing it out. (its usually more runny too if its fresher like hasn't been sitting int he bowls geting all dense)
the body also does this if something you ate upset it, its like a defesive mechanism to flush out quick.
Theres 2 reasons the body would flush itself.
1. you ate something it didn't like or couldn't digest properly. Thus it got it out of your system ASAP
2. In nature a few species will actually eat like crazy thne either throw up every thing they ate or quickly pass it all out so that they can eat more right away. Basically the body leeches the most important nutrients out of food first. So then after the body or animals spits it all back out so the animal(shark) or what ever can eat all over again and reap the top end nutrient all over again. (they don't do this often only when food is all of a sudden extremly available when it is usually not so)
So yes if you ate massive eccess all the time the body would probably increase your bowel movements and not digest everything at every meal, however in order to keep that effect constant you would have to eat learge meals all the time so that the body knows its getting food again before it can even finish the last meal. you obviously shouldn't take this approach because in order to get to this point you will be putting on fat at an alarming rate being well over the excess calories needed to promote muscle growth.
Trying to do this for just one meal a day will not work, it may at fitst but eventually the body will learn to hold the food untill it can be comepletlely digested.
This is one of the reasons laxatives were used for wait loss, you could eat alot and force your body into the state where it dumps the food w/o completely digesting it all however this is not a healthy practive by far.