Bmi Baby`s charge £15 charge for exit seats is just another way of pocketing a few extra quid for what is after all a legal requirement to have pax in these positions. I think you will find that the policy changes very slightly at 20 mins before close of check-in when, if nobody has paid the ransom, the seats are allocated freely. So if you have paid your £15 it`s quite possible you`re the only person who has.
As for the size of the person in the seat I`d rather see someone there that was gonna be out like a shot and not some lardy boy that can`t get out when it`s open.