I think this issue kicks off because overeating is normally a vice that only affects the person doing it. Unlike smoking, where it's equally distructive to your personal health, for years we've felt it acceptable to make negative comments about smokers and limit their options because their smoke affects the rest of us. However, public transport is really the only time that someone else's decision to overeat affects you - by incroaching on your space. Unlike smoking, or drinking, if you overeat, the negative concequences to other members of the public can't be stopped by just not doing it at that moment, if you are fat, you are fat all the time, if you are a smoker, you can just not smoke for a few hours.
OP, the people you sit next to huffing and puffing aren't fat-haters or fat-phobics, they are people who are suffering negative concequences of your lifestyle choices - that is, you physically taking their allotted space on a plane. Your choice to overeat (which may have emotional reasons for it, and not be entirely in your control) means they have the quality of their plane journey lowered, it's not their choices that have lead to them suffering a lower quality experience, they will have paid just the same as if they'd been sat next to someone who was size 6.
Often, it's the point your choice affects someone else that everyone else feels they have a right to comment negatively on your choices.
In the same way people who want to keep smoking, while knowing how bad it is for their health tend to put in elaborate rules that make their lives harder in order to minimise the effect on other people (such as only smoking outside regardless of the weather, showering and cleaning teeth after each fag before touching their DCs, not smoking in cars - even if that means making regular smoking breaks making journeys longer etc), in the occassions that your size effects other people you should be the one to put in plans to limit the negative effects on others - so if that means between you and your DH you buy 3 rather than 2 seats os you can have a spare seat between you, or you fly premimum economy so there's extra room, or you make sure you're at the airport an hour before everyone else so you can get to check in first and get seats together - then you do that. It should be you and your DH that shoulder the negative concequences, not others.