Just like you said, it is for improvement of BMs. But, it is not just small servings, but frequent small servings, as you want to maintain your nutrition.
In addition to helping with function early on, it also helps with nutrient absorption. In the first year or so while your gut is adapting to life without a colon, it is easier to overload your system with meals that are too large. This can increase diarrhea.
The size of a small serving is subjective. You'll find this to be a common recommendation for pretty much everything pouch related: experiment to find what works for you to improve your function. You will find that you can eat more of one type of food without impacting your fuction, while another needs to be more strictly reduced.
The good news is that regardless of what you do now, things will gradually get better. So, try not to make yourself crazy with adopting the "perfect" diet and eating patterns.
Jan