It wasn't clear in the OP whether the two larger people were traveling together or not. I suppose they were. I would imagine it's as difficult for them as for their neighbor. I had some ideas for them:
* try to get into a 2-across seating aisle
* buy 3 seats for two people if you know you have to be in a location that's not 2-across
* don't sit together; try to get two aisle seats in different rows that have minimal restroom traffic
It could be worse: on a trip home from Rome to LAX once, our seatmate opened and ate an onion sandwich soon after takeoff. I am not kidding! And nearly every traveler has endured a seatmate who considers bathing a rare treat.
And Phydeaux probably doesn't believe this, but obesity is not always a "lifestyle choice." Medical conditions can contribute. And different people's bodies process food differently. I know families where (for example) only one of several children was way above average size by the age of 18 months and still is, yet they were all raised and fed identically. Other conditions can make close quarters unpleasant for neighbors, too, but we're all in this together, especially on airliners.
[My prediction for the future is that we'll travel anesthetized in horizontal tubes, kind of like the pods they show on those interstellar travel movies, only much closer together. Just think, they could load us into a 3x3 matrix of those in the terminal, and then convey them automatically into a cargo-type aircraft. No need for entertainment, food, or restrooms! I should patent this idea before Ryanair thinks of it.]