I have had similar experiences flying regionally from Brisbane on Bombardier Q400. While the Q400 is one of my favourite short hall aircraft, they have tight seating. Many a time I had to sit beside a big miner returning home/to work. I am tall (192cm) myself so having to sit beside someone who is either very tall as well or obese and taking up more then their seat is very uncomfortable/challenging.
I don't blame the airline as they are not responsible for the size of their passengers. I blame the passengers as they could be a lot more considerate and think about others when purchasing tickets (namely buy premium economy or above if they can't fir into a standard economy seat) or selecting seats (say exit rows or bulk head seats where there is significantly more legroom relieving some of the pressure on neighbouring passengers).
As I am relatively slim and tall, for long haul flights I try and get seating that I am more comfortable and also often make requests when booking tickets/checking in. Sometime it mean trying to check in as early as possible or turning up to the airport very early to try and see if more suitable seats are available. I tr and be considerate of others and won't protrude my body parts into their seating space (unless I am travelling with my family to which I often do to stretch out - it is not often to have us all sitting in exit row or bulk head seats so we tend to travel like the other economy passengers so that we can stick together).
I have noticed that some airlines such as Turkish Air and Southern China mark me automatically as a tall passenger when checking in at the airport...this marking automatically flags me as a tall passenger for other (connecting) flights on the same booking. I am yet to see an Australian Airline do the same.
Anyway, it is the responsibility of passengers to be courteous to others and if one is big or extreme tall, one needs to factor this in when making ones own bookings and seat selection. If it mans paying slightly more, it will mean that not only fellow passengers will be more comfortable, but one will also be as well.