All this "coughing up" crap sparks of elitism.
I don't agree with you, El Grifo.
It is a matter of paying for the service that you wish to receive.
Buying C should mean that you have a guarantee of more space, but it comes at a price differential.
Buying Y means you will likely have less space, but that is the choice you make.
As to whether the airlines are liable, I don't know.
Although the contract probably does not even specify what an economy seat is, there is a thing called 'implied terms and conditions of contract' that may cover this.
I'm not a lawyer though, so don't have an opinion whether it does.
Bravo to Southwest Airlines for having a fair policy (buy two seats, get refunded for one if the flight is not full.)
One can also buy two seats as a 'person of non size', as I do with Ryanair if using them. As there is no premium class, having guaranteed space is worth every cent and makes a 3 hour flight much more comfortable.