Why is that I eat a lot but my poop is small?

Like I thought the amount of food we eat is the same size of our poop when we poop. But maybe I was wrong? by the way sometimes I eat a lot and walk 10k steps and rarely poop too is it because is it breathe out in lungs? (I read somewhere but forgot) Or... something else can someone explain?
Why is that I eat a lot but my poop is small?
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