I know for myself, the external cue of "there is still food on my plate" is a strong one, since I was raised to always finish my plate. I now do so religiously, but try to set myself up for success by getting smaller portions :).
I also do not like to waste as I have actually seen people in some parts of the world who are malnourished and always hungry... I can't leave food on my plate for that reason and I wish many restaurants would just server me less food. I'd still pay the same price.
2 options: Specifically ask for a half-sized portion, or eat half and take the rest away for lunch tomorrow.