We often found that we still left food on the plate, even when we shared. The food is actually the smallest cost in running most restaurants; many chefs provide enormous serves to attract customers.
When I travelled alone, it was more difficult. I, like many, was raised in an environment where waste was frowned upon - waste not, want not. As a post-WWII child I was taught to clean my plate before leaving the table.
It takes discipline to break the habits of a lifetime and leave over half the food on the plate when you know you are paying for it. But if you eat it, you will pay much more eventually. Specify that you absolutely, definitely do NOT want chips/fries. Many restaurants add them automatically.
For dinners, when travelling alone, I found the method I used most often was to order an appetiser and a side salad instead of a main course. If that was too small I would order a second appetiser. That led to some marvellous and delicious meals; often the appetisers are more varied than the main course and aren't accompanied by piles of potato, rice or fries.
My most common lunch when travelling is "Soup of The Day". But be a little careful of thick "cream" soups; they will often be thickened with cornstarch, flour or potato. In 2006 I happened to be wandering through Germany in Spargelsuppe season. Bliss.
I'm sure I'll recall other tips after I post this, so I'll probably return regularly and up-date it.
Everything in Moderation - Except Laughter.