Plate size does really help with portion control. Seeing you enthuse over your bentos has made me rather jealous, I'm going to have to get some.
When I eat out, I always ask for a to-go box when they bring the food. That way I pack up half the food (or whatever is beyond a reasonable portion) right away. It's so much easier to eat proper portions if that's all that's on your plate. Especially when you were raised to "always clean your plate."
Plus, then I have another meal already packed and ready.

Never been to Ikea, there isn't one near me. But I have wasted rather a lot of time on their website.