If you do not require the extra seat due to your size, then you don't get the extra seat. I had one instance when I had my purchased extra seat and someone tried to sit in it because families had put their lap child in the seat next to them, and the last people to board did not have seats. The FA came to me and checked my boarding passes, then went down the aisle, checking every family with a child under 2. Sure enough two families had set their kids down (and without proper harnesses!!). This is exactly why I purchase my seat. I need it and I want to make sure that it will not be taken away from me.