I haven't seen this mentioned anywhere in the forum; if it's there, I couldn't find it. In the moodlet section, the Guide says: "Unfortunately I didn't see that leftovers gave this meal boost. Leftovers are simply more filling meals than the quick meal option, and just as quick to prepare."
In my experience, leftovers always give the same level of moodlet as the original meal. Unless you are using the cheapest fridge (I think it's called Chillgood) - with that fridge you can get a "tastes like fridge" negative moodlet from leftovers. My sims live on leftovers, at least until they can accumulate enough LTW points to get a food replicator.
