Also could be they are just constipated, and the stool will build up to a large size, until it comes out. I'm sure you are familiar with how to ease constipation, but as a reminder, they should be getting plenty of exercise, drinking plenty of water, eating plenty of fiber type foods, to include things like cheerios and 100 percent whole wheat bread, and at their bathroom times they should not be rushed.
Were they constipated as babies and as young kids? There is a condition in which the colon can be somewhat enlarged and that can produce larger than what would be considered normal bowel movements.
I once heard of a medical condition called "Wide Anus Syndrome." Basically, I guess, the hole down there is bigger than normal or something and causes large stool. It's something you might be able to perform an internet search on and find some more answers. Good luck!