I'll bite. Airlines are a great example because there are really strong physical constraints on flight, but the basic rules apply to almost every piece of built infrastructure. What does it take to make a plane "accessible" and what standards will it be built to? Are we going to accomodate "small fats" up to 300 lbs or so, or will we continue into the 500 lb range or 700 lb? This matters because aisles, seats, and doorways will all need to built to standard.

If you've seen the "Big Johns" in Vegas you'll know that the washroom alone will take up the entire width of a small passenger jet. That will allow for the oversize toilet, room to turn, the doorway and aisle. That means there will only be one unless we turn them sideways to put in two. But those toilets now remove 6-8 rows of seats. So that's 18-24 fewer paying passengers. I could go on here but you get the idea.

Widening the aisle would require removing 1-2 seats per row. And the remaining seats become wider so there are now 3-4 people per row instead of six. So the economics really matters here.

These discussions are true for every piece of infrastructure. It's not just a matter of making things bigger to allow people room to move and sit. Every supporting piece of infrastructure has to match. What does it do to land use if parking spaces need to be 50% wider to accomodate larger vehicle doors that swing fully open?

The built environment is a series of interdependent systems that are built to a set of standards - some tightly regulated and some informal. Changing those to accomodate a larger body size is not a simple task.