I guess that means you are staying in rooms with refrigerators and microwaves? In my experience, portions in Europe are not as large as in the US. But I agree that a plate-splitting charge (which exists, but isn't that common in the US, especially if you order drinks or a bottle of wine, helping out the restaurant's earnings on the meal .... ) is very unusual in Europe.
Because each restaurant has its own business-model, quantity of tourists, and personal experiences, I don't think your OP question can be answered definitively. That's especially so for Portugal, since there are still many important destinations there that are not overrun with tourists.
An important wrinkle is that in most European countries, meals take much longer (at least for locals ... ) than they do at home in the U.S. A restaurant that takes reservations in Europe would almost never, never expect to "turn" the table and seat another party during a single dinner hour. The reason I mention that (and it's a much less disputable "internet fact" than whether a doggie bag might be acceptable) is: If you make a prime-time reservation at a moderately famous restaurant, and end up ordering a penny-pinching meal, you have done them a disservice, and perhaps, slightly, been an "ugly American."
That sounds harsh on the internet, and of course I can't criticize you for something you haven't DONE! I just wanted to consider the point. At a walk-in, in a local place, your early departure might surprise them, but of course they can then take advantage of the unexpectedly open table.
I would suggest that you concentrate on appetizers and small plates, which should permit you to explore the menu without ending up with more than you can eat. Leftover food is rarely as good as the first time, anyway.