James 1077
easyJet do have a policy that if you can't fit into their seats then you need to purchase another one.
This was one of the reasons I asked the question to lowcostdolly and why I worded the question so specifically to say that the slim person could not fit in the seat without bodily contact with their larger neighbour.
It is one thing to have a policy and another thing to define what 'not fitting into a seat' looks like.
From lowcostdolly's reply, I would deduce that easyJet have not issued a clear definition to 'operationalise' the policy, so that places the crew in a difficult position, unlike clearly defined criteria about who may sit in an exit row.
I have not been able to find a Ryanair policy about this matter, but the company does offer two ways of securing extra space
Ryanair - FAQS : Can I purchase an extra seat for a large person?
Ryanair - FAQS : Can I check-in a musical instrument or purchase an extra seat to carry it in the cabin?
As times move along, people get bigger and flights gets busier as theworld comes out of recession, I anticipate legislation to resolve this matter.