Speaking from a position of utter ignorance
I would assume that various models of plane have a maximum weight they are permitted to carry - probably set somewhat below the point that safety would become an issue. If the combined weight of people+baggage exceeded this, wouldn't it be fairly likely that a few passengers+associated baggage would have to get off, just to get back below the numbers insisted upon by insurers (or whatever)?
( Happy Phantom has been to Hastings, Brighton, and Eastbourne too, Fri 28 May 2010, 14:59, 1 reply)

but if the aircraft was skirting that close to the maximum weight that 4 people would make a difference
then I'd rather not fly with them
( likeajackhammer, Fri 28 May 2010, 16:33, closed)