Here's the background of the word COLLATION:
It comes from the latin cuм (with) + lectio (reading)
Basically it's supposed to (originally) be a small snack taken during a substantial spiritual reading read to the monks in common.
The amount of food involved vs. reading is shown by the fact they called the meal a "with the reading". Food was basically an afterthought to what was primarily a spiritual reading.
The monks (who obviously fasted a lot) would have a glass of wine during these readings, to give them some nutrition/strength for the days' work, prayer and study. Something besides the one main meal.
But eventually they added a crust of bread so they weren't drinking wine on an empty stomach.
So think about that as you decide what to have for a collation. It shouldn't be mistaken for your main meal, not ever.
But at the seminary we didn't weigh anything either, and no one judged their brethren on how much the others ate for their collations. Each person's body is different. What is adequate for one man would be insufficient for another, and over-eating for a third man. That's why the specifics of fasting were left between you and your spiritual director.
However, the seminary encouraged a certain amount of fasting by having cold cereal OR bread/peanut butter/preserves for breakfast (normally, both were served every morning), no dessert, and the evening meal was meatless (our main meal was always at noon).
Another thing no one brought up yet -- it's hard to pig out on lentil soup. You can only eat so much of one food, especially one that's pretty penitential to begin with, like bread, lentils, beans, etc.
In the realm of food, variety always encourages (and allows) you to eat more. That's why people end up so full at Thanksgiving -- there is so much food, and all of it is desirable.