The report citing SEN as London 's second most expensive airport was I believe produced by CAPA. I believe I may have only seen abbreviated version but the conclusion as I recall was based on an overall cost per passenger basis, in which case it is possible the methodology would have skewed the results in favour of those airports able to handle larger capacity aircraft.
If anyone has access to the original report it would be useful if they could examine the original and post their assessment here so that we are not relying on unsubstantiated heresay.