Hi cj, I am about 145kg at time of typing and I fly all the time. I mostly need to get a selt belt extension, I either ask when I get on the plane or press the attendant call button when I sit down and ask for one, or I go straight down the back if one of the attendants is down there and ask for one. I don't try to be discreet about it as I am quite obviously a big person and it all seems a bit silly, but to each their own.
I haven't been able to use a tray table since time immemorial and so tend to skip food but then I only really fly nationally. When with the beloved beanpole that is my husband, we put the arm up between our seats and I get a bit of his, and we share his tray table.
It's all pretty OK really, the level of service or willingness to assist with 'comfort seats' which means an empty seat next to you, is not as high as it used to be and definitely Air NZz are better for this kind of thing. If you ask rally nicely at check in and don't sound like you expect it of them, they will try to put you in a row with an empty seat beside you. This is a good reason to get there early! Mind you with online checkins etc, this is also harder than it was, but I have experienced it when they will move other people who are checked in, to help me, because, I am sure, I am always pleasant, smile sweetly, ask humbly ensuring to stress that I appreciate they don't have to help me but if they could I would be very grateful etc. And it's true, I AM grateful!