Instead of ordering the best value, order the size you actually want.
I imagine that my experience is similar to yours. When I go to order food, I am mentally doing a quick calculation between the different sizes to identify the best value. If 20oz is only 50¢ more than 16oz, it is nearly impossible to order the smaller size. Even when I actually want the smaller size.
This leads to many different problems. Instead of being satisfied with my order, I have to choose between eating to discomfort, or grappling with the feeling of having wasted food. Often times, I will eat more than I wanted so that I am taking full advantage of the great value that I got.
I remember a particularly poignant example of this when eating at Smashburger one day. They were having a promotion on the Triple Double which made it even cheaper than the regular burger. I did not actually want that much meat, but I felt compelled to order the bigger burger for cheaper. My friend came behind me and ordered the same burger, but asked for only a single hamburger patty. I was struck by the brilliancy of his solution. That had never even crossed my mind.
As I have been on my healing retreat, I have seen a few movies in the theater. I used this as an opportunity to practice the new approach I wanted to take. It is a real challenge for me to pay $4 for a smaller bucket of popcorn, when the large is $5 and comes with a free refill. The thing is, I can never eat that much popcorn without getting sick. So I have forced myself to order the smaller size and stop worrying about the value I am missing out on. And I have enjoyed my movies much more with just the right amount of popcorn.
There are many situations in life when we make choices based on the perceived value of the options. Instead, we need to become more comfortable with ourselves, and more aware of our desires. And then be willing to act based on what we actually want, not what we think we should want. This is a much happier, and more comfortable, way to live.