Upholstery does not in itself create comfort. Traditional coach seats gave the illusion of comfortable padding but were angular, not reflecting body shapes. The critical measure is not the size of the seat but the space allotted to each seat and the space between seat rows, called pitch. It’s a slippery term applied to the distance between one seat back and the next. The bulkier the seat, the more of the space it swallowed within the designated pitch, leaving less for your actual body.
If the pitch remains the same but the seat becomes slimmer, the result should be more body room, right? If only. The newest coach seats drop the upholstery and, instead, are shells molded to the human spine. In theory, they save weight and add legroom. In practice, though, the carriers use the space gained by slimming the seats for jamming in extra rows of seats and decreasing the pitch—Jet Blue’s seat pitch will, for example, fall from 34 to 33 inches.
This kind of density isn’t limited to single-aisle airplanes flown by budget carriers. For example, the Boeing 787 Dreamliners flown between Japan and the U.S. by All Nippon Airways, an airplane that in other respects has the benefits of advances in the quality of cabin air and lighting, have a 30-inch pitch in coach with hard shell seats that do not recline… for flights that can last longer than 12 hours. In the same cabin, the business class has flat beds with a 70-inch pitch.
Then there is the other critical dimension, width. The narrowest seats in coach are generally 17 inches wide, although some airlines have pared that to 16.7 inches—all this at a time when the world’s bums are getting broader, not slimmer.
A good resource for helping decide on flights and seats is Seat Guru.
Colleen has very long legs physically can't put her feet on the floor in most coach seats. Gate agents once made sure she had an aisle or bulkhead seat, but that went away when airlines started charging more for extra coach legroom. Some airlines have a policy of making heavy passengers pay for two seats or pay for a class upgrade. Expect more people to be required to pay for upgrades because they simply won't fit.
An interesting size related issue I didn't consider before meeting Colleen was car interior size. Car interiors have to accommodate a sizable percentage of the population. Models have differing limits, but most try to safely fit the 15% female through the 95% male, although this varies widely. Beyond these limits can be unsafe, so unusually sized people either take their chances or have modifications made once they run out of choice. The cars with or without room can be surprising. Some very small cars like Minis and Smart Cars can have very large amounts of legroom - not having a rear seat helps. On the other hand, many SUVs and pickups have limited room and Colleen can't fit behind the steering wheel. She'd make a poor car park attendant.
Crash testing is done with a very limited size range of crash dummies. This is improving, but there is still the issue of having airbags that can judge the position and size of an occupant to deploy to the right size in a collision. People have been killed by airbags for being the "wrong" size. A designed based health hazard for being far from average.