That man was an @ss. There is no need at all to throw a tantrum or even complain within the larger man's earshot. What a prick.
That said, it is unfair to pay for an airline seat that is already small enough and then lost half of it to a person who is unable to keep themselves within the limits of their seat. I flew Sydney to Brisbane and sat next to a lovely lady who was extremely obese. She had to ask for an extension seatbelt and the arm rest could not go down between hers and my seat. It was so embarrassing for her, I'm sure. I pretended like nothing was wrong and just started to chat with her and act normal etc..but in actual fact, it was hell. I was on the window and was literally shoved up against the side of the plane. She was actually sitting on me. The plane was full though and I felt too bad for her to say anything to anyone..there was no way I could have anyway, without saying it in front of her and I wasn't about to do that. It was only an hour flight so I put up with it.
If I was on a long haul flight, I would have had to say something. I have no idea what they can do when the plane is full though.
I might be shot to pieces for this but I DO think that if you don't fit within your assigned seat..you need to pay for a second seat. As embarrassing and unfair as that seems...I still think it should be the rule. Regardless of cost, if you are obese and want to fly..well you need to pay for it. Just like as I have a chronic medical condition and want to travel, I have to pay an enormous amount for travel insurance. It's not my fault I have this illness and it might not be fair just as it may not be the fault of the obese person that they are bigger and while it's not fair..that's life.
That is the only solution I can come up with. The airlines are not going to change the size of their seats. It would be nice if they charged the second seat at a reduced price though, given that the person won't be eating 2 lots of meals etc.
But yes, they should have to pay.