Incidentally I eat out a lot and I can tell you several strategies I use:
(1) You can get sharing platters and just not eat a lot (ooh yes, I'd love some garlic bread, who'd like to split some with me....no, no, you have some more, it's a bit salty for me)
(2) The waiting staff don't give a rats arse if you eat only a third of your meal and frankly if those dining with you make a comment on it, say 'feel free to eat it, I think the portion sizes are massive and I don't intend to be a lardarse because someone decided to buy oversized plates' or just treat them to a killer stare combined with a 'really, well I've enjoyed what I've eaten but I'm full now, you wouldn't want me to feel sick would you?'
(3) Doggy bag - not for the dog/cat. Most restaurants are delighted if you say 'this was marvellous but I'm absolutely stuffed - can you wrap it up for me? (I've been known to take a tuppaware pot to the curry house because I eat about a third of it and it does reheat pretty well!)
(4) Go for big meals with lower calories - soups, salads etc. Or pasta is good - you can easily cut the pasta dish in half with your fork and tell yourself that the rest is going home in a box with you, then you have technically finished it.
(5) Biggest one - GET OVER YOURSELF - if you look at the serving in front of you, that is designed to sate the appetite of a 20 stone bloke! You don't need or want to eat that much. It's okay! It's not compulsory to clear you plate. This is meant to be a pleasurable experience.