[lkbm]

Skinnier would mean more surface area to volume, but less surface area total.

If we're just looking at energy in and energy out, it could partly be less thorough digestion. I wonder if there are poop studies that find more leftover fat or carbs in faeces of children.

[layla5alive]

Yes - for example: overconsume fat to an extreme degree and watch the color of your stool turn towards white/clay, and watch it start to float, among other notable changes (size, shape, consistency, odor, etc.)