@dyslexia,
You know what - I always get screwed when it comes to armrests. It seems the over riding theory is the larger person always get the "shared" armrest.
So say you are sitting in the middle and you are petite. There are two chubbies on either side - not only are you screwed because they overlap into your seat area, they feel they have a right to arm rests on both sides. If you are on the aisle, and a larger person is in the middle, they will take the arm rest from you.
My opinion, the person in the middle should get both arm rests on either side of them. Since they are in the worst spot squeezed between two others, they should at least get a little bit of benefit.