Hi OP,
I used to be a very frequent business flyer. Admittedly, in later years I've been in the business/first class sections, I spent years earning my "wings" as it were in economy.
My biggest gripe was inconsiderate passengers.
I've sat next/between all sorts of people.
The ones that stand in my mind for negative reasons are not those who were overweight.
My personal experience is that people who are overweight are very aware of the space they taking and try and keep within their "zone".
On the other hand, men (and it's always men) spread bloody everywhere however slim they are. Legs/arms encroach in "my"
space. Elbows everywhere. No sense of personal space whatsoever - actually not true, they seemed pretty good about not encroaching on the man on the other side but my space was fair dibs until I made clear otherwise.
Worst are drunk passengers. I've had a women swigging duty free vodka next to me and vomiting in the footwell over my shoes. That was a highlight....(the cabin crew were fab though and really looked after me).
I've flown next to some people much bigger than you and frankly I'd rather sit next to a considerate overweight person than a manspreader or a drunk or a person that wants to chat endlessly, or the farty person, or the person constantly praying we don't crash, or the person who thinks you can do seat yoga, the parents with a screaming child they don't try to console....I could go on.....
I guess where I'm going with this is that you wouldn't even hit the top 10 of people I'd not sit next to on a flight OP - unless there's something you're not telling me 