Dr Ralph's suggestion that one should 'litigate' in these circumstances is not good if our society wants to minimise the costs - because there is a huge cost to us all - in USA style litigation. Litigation is unlikely to result in airlines suddenly ensuring magically that seats become three centimetres wider, or the pitch alters by five centimetres in passengers' favour. All it does is push up costs and hence fares that we pay on all four domestic airlines, and some fares have been rising faster than inflation in the past year so 'encouragement' for this to further occur would hit consumers and non-airline businesses.
However the QF onboard staff attitude was very poor. I can well imagine how 'passive aggressive' the QF staff would be, except for the airport manager who knows jolly well that the OP was being quite reasonable.
Granted that with OLCI and many passengers wanting to sit together (married couples, friends) the situation is complicated, and few domestic airlines (bar if there were such flights in Tonga!) know the weight of a passenger before he or she presents at the gate (especially now that bag drop and so on is increasingly automated) but airlines should be trying to give these obese passengers an aisle seat - although as one who likes aisle seats, if there were 50 obese passengers on a flight and I missed out, I could be unimpressed.
eastwest101's balanced comment hit the nail on the head. A difficult issue but the OP did nothing wrong that I can see. Being made to perceive by QF onboard cabin crew that he was 'the problem', not the obese man next to him, was most unfair.
RedRoo, do you have a view?