Care to explain why exactly?

I've always been thaught - ok, doesn't mean it's correct, or related, which is why I'm asking - that one difference between meat and vegetable eaters is the length of the large/small intestines: shorter for 'pure' meat eaters like cats, longer for herbivores like cows. And somewhere in the middle for humans as they are considered omnivores. Reason for this would be that it takes longer to digest/break down plants, which I assume is done by the intestines to extract nutrients for it?