Restaurant food makes me want to pig out. It's engineered to do that, to make you crave it and want to eat more of it. Our bodies evolved to love fat and sugar and salt, and we still chow down on it when we can get it, even though the food we have access to has far more than we actually need. We can eat too much before we feel satisfied physically.
I think my office's weekly company lunches have been a bad thing for my waistline.
Most of the time we get Indian or Thai takeout, and I found that I would eat the whole thing before I realised I wasn't hungry anymore.
My current tactic is just to not eat out of the styrofoam box. I take a bowl, I put half the food in it, and I eat. When I get to the bottom of the bowl, I ask myself if I'm still hungry. Usually I'm not, and I have lunch for the next day too.
One thing this made me realise was that when I was eating my entire lunch, I was actually feeling bloated and drowsy in the afternoons. By not eating more than I need, I actually have energy in the afternoons.
It would be hard to do with a sandwich like a Big Mac, because it's relatively small and concentrated (not to mention would probably taste nasty and look worse the next day!). But if you get things like curries and stir-fries (which aren't always healthy foods -- aah, coconut cream curry, my tasty and saturated arch-nemesis), you may find that "one serving" is actually two or three.