As long as it isnt against any rule, I don't see the issue.

In J/F, you get your meal. If you don't like it or can't eat it, they offer what's left on the plane (usually the economy stuff or the snack boxes). If you want more, that's probably outside the standard offering and up to discretion. Some FA's want to make the passenger happy so they'll oblige. Other's don't. It never hurts to ask, but one should expect more than they paid for. If you get your single meal or snack box and decide you want to eat it later, I see nothing wrong with taking it with you and consuming it later.

In the lounge, it's a different because the food is unlimited. Most have the food for you to consume in the lounge, and not to take things from the lounge. Some even have signs stating this. If I'm going to be stuck in economy for the next 12 hours with no edible food, sometimes I will take a sandwich or cookie from the lounge. I don't think I'm taking an excessive amount but I recognize it is wrong to do. If I took a grocery bag and just started dumping food in there to take, I would find this to be excessive. Others lounges specifically encourage it, obviously it's ok in these lounges.

Sometimes when you live in a society and have to be in public, there are certain ways one behaves and one learns what it generally acceptable and one isn't. If it isnt acceptable, generally someone will tell you.