I've read enough articles about the benefits of fruit and veggies - even for canines - that we "plan" for whole-prey-raw, but then use fruit & veggies as treats and "adders". I don't plan for the fruit & veggies, but Denna gets quite a bit just by watching me cook the humans' dinner every night - trimmed pieces go in her dish before the compost pile.
And she loves it all, too...
I have also noticed a LOT of the fruit & veggies come out in the exact same form as they went in... so there definitely is some truth to the non-digestibility of many forms of vegetable matter...
And, if you want to bulk up the poop (getting back to the original posters subject matter)... just add non-meat-material to the meals! You'll see it all come right out the other end!