Could the US government force airlines to make seats wider?

Definitely a subject needing investigation. IMO, US domestic economy seats could be 20% wider with pitch increased from 31 to 37, especially for passengers requiring the extra room on those 4-5 domestic routes. Airlines will see red with this of course, but then again the average passenger over the past 5 decades since the introduction of the 707 has increased in size on a global scale and to counter this airlines simply charge the percentage to cover the increased real estate required to achieve this. 

Am sure most passengers would not have an issue with this, especially when they feel the circulation return to their feet while on those long routes being traveled. This also applies to US First class domestic seating, not much better then economy when it comes to width and pitch. Shall we look at american First Class and United Basic economy seating as a perfect example. Health before profits!

Hmmm.  Certainly an interesting take by the Obesity Medicine Association, but perhaps the solution is to replace the last row of (typically) 3 seats with two (2) purposely made 'bariatric' seats for those pax with genuine obesity challenges, and allow them to board the flight first as is often allowed for others pax with mobility issues (including by own aged parents).  

Nice idea but the most common US domestic plane is a 737 (a 1960's design) and its fuselage is smaller than an A320 and thus seat width is governed by the available space inside the airframe. Cannot see this getting much traction. I can see there being a zone with fewer seats across for those who are on the larger side.... 

At what stage does equality come into play, if an obese person gets a bigger seat, why shouldn't a thinner person. If a thinner person has to pay for excess baggage, should this be standardised against the weight of the person? Buy a second seat or lose some weight, there's the options. Plane travel has never been more affordable and open to all demographics, airlines are in it to make money and until you bring out a government operated Not for Profit airline i don't think there'll be much leeway here.

The FAA has been dragged kicking and screaming to finally get involved in this, they were told, bluntly, to investigate this issue and rule on it three years ago.  Unfortunately, too many close connections between the regulator and the industry meant nothing happened.   Given airlines have already placed orders for aircraft that won't be delivered for several years, obviously any mandated change needs to be several years ahead, however it should not take forever.  With respect to all those 737's and other narrow bodies flying, there should be a directive that say within 2 years 20% of the airlines fleet has to be converted to 3 - 2 seating, (as on an Airbus A220,) and those planes must be used on the longest flights offered by that carrier, and the percentage changed over is then ramped up to be completed say within ten years.  

Prices will have to increase, possibly resulting in less people flying, but maybe that's not such a bad thing given the current environmental issues, or the problems caused by over-tourism in some places.  

Many of my flights have been with my brother in law who lives with Down Syndrome. He can only sit with cross legged as his feet don't touch the floor. (Dangling legs for many hours is painful.) 

Navigating the aisles, accessing the toilets or adopting a safe brace position when his feet don't reach the floor are issues when flying with him. 

I suspect the range of solutions may end up being more than wider seats. 

Bigger seats=fewer seats per plane= more expensive seats

Or 

Just pay extra for bigger seats as you can now. It’s called premium economy or business 

Without mandating a particular seat size, I would like to see a requirement for travel agents and web sites to show the seat dimensions (pitch and shoulder width) when booking and searching. This would help enable customers to ensure the product is fit for purpose in their specific cases.

what is the scope for making armrests with narrower bodies to account for the wider bodies between them?

(Personally I'd be much happier with greater room for knees, the stuff that usually makes people wider is able to be squished into tight spaces - whereas femurs don't really squish)

Correct that bigger seats will only happen through regulation, as otherwise competitive pressures dictate cramming everyone in.  ie lowest common denominator, or "race to the bottom" if you like.

In the meantime, if you need it, buy a bigger seat or go by bus.

Perhaps the U.S. gov't should address the actual unstated problem: that the majority of Americans are obese. This is tinkering around the edges, window-dressing, dodging the problem [pick your own metaphor]. The fact is, larger seats have always been available for 70+ years. You just have to pay for it. Gov't policy should fight obesity, not normalize it! That's not being a 'fat-shamer' or elitist; that's just being plain old sensible and health conscious.

"The mandate does not include passenger comfort." - thus we already agree that flying in those seats is uncomfortable and we already agree that no-one cares about this. Now we talking about safety and physical pain while using those seats. Yes, aviation has changed in last 3 decades and IMHO changed in wrong direction.