The U.S. Air Force is satisfied that modification to the US16E ejection seat used in its F-35A fighter has been modified to deal with a weight problem. Not the weight of the ejection seat but the weight of pilots who can use it. No pilots weighing less than 62 kg (136 pounds) have been allowed in F-35As since 2015 when the problem was discovered. Ejection seats, like cockpits in general, are built to accommodate a limited range (in terms of weight and size) of pilots. The new F-35A ejection seat required some expensive modifications so that it could safely and reliably handle pilots weighing less than 62 kg. Actually there is some risk for any pilots weighing less than 75 kg (165 pounds). Some female American fighter pilots weigh less than 62 kg and would be subject to injury or death of they used the unmodified US16E. The air force was under a lot of political and media pressure to spend whatever was required to accommodate a handful of (mostly female) pilots. The manufacturer (Martin-Baker) has developed a fix but revealed in early 2016 that it would not be in service for about a year. The air force recently tested and verified the US16E modifications. That is good news for the cash strapped air force because if the fix did not work another manufacturer (UTC) gad a new seat (Aces 5) that handles the problem and could replace the Martin-Baker seats although that would be even more expensive. Meanwhile the hundred or so F-35As with the unmodified ejection seats will have those seats upgraded at the rate of about 14 a month. So by 2019 all F-35 seats will be able to accommodate pilots weighing from 46.4 kg (102 pounds) to 111.3 kg (245 pounds.)
Historically aircraft, and other military equipment, were designed to handle people of only a certain weight and size. The most obvious example is plate armor and mounted troops in general could only serve if they were not too large or too small (or too heavy). In Russia (and a few other countries) this extended to armored vehicles, especially tanks, where only the short of stature (and muscular, especially if you were the gun loader) could serve in tank crews. The restrictions were more common in aircraft, especially fighter aircraft. This was never an issue before but now it is. This is more about politics than military necessity.
This is not to say that some items of military equipment are not constantly being updated to handle a wider range of personnel. Modern fighter ejection seats are constantly being upgraded to deal with new pilot equipment and to improve reliability and effectiveness in general. A recent modification has been made to handle the heavier HMDS helmets. Passive arm and leg restraints have been added to greatly reduce arm and leg injuries when limbs are not kept close to the body during ejection. Some seat models have added a new parachute design that descends more slowly thus reducing the number of parachute related ejection seat landing issues. There are also new rocket motors that automatically adjust for pilot weight. This is particularly important for female pilots. The rocket motor controls and stabilization system makes it less likely that the ejection seat will hit the tail of the aircraft or land at a bad (for the pilot) angle. Newer ejection seat designs are also modified so that they are much easier to install and remove from aircraft, saving several man hours per removal or installation.
Since World War II over 10,000 aircrew have successfully used ejection seats (mostly of Western manufacture). Very few have died in ejection seat related accidents but when that does happen is causes consternation among pilots and concern among ejection seat makers. Russian and Chinese made seats have proved to be nearly as reliable as the Western ones. But all ejection seats are vulnerable to poor maintenance, which has been found to be the most common cause of ejection seat use fatalities.
Ejection seats costs between $200,000-300,000. Most ejection seats weigh about half a ton and are complex bits of technology. There's a lot that can go wrong but rarely do you have accidents if the seats are maintained properly. Ejection seats became essential as military aircraft became so fast that a pilot could not safely climb out of the cockpit and jump. With the higher speed, there was the danger of hitting the tail. Also, escaping pilots were often injured or stunned and unable to get out quickly enough.
The first ejection seat design was developed in Germany where the seats were first installed in their He 219 night fighters during 1943. These used compressed air to propel the seat out of the aircraft. A year later rocket propelled seats were installed in the He-162 jet fighter. By the end of the war, all of Germany's jets were equipped with rocket propelled ejection seats. While the Swedish firm Saab had also developed a rocket propelled ejection seat it was British firm Martin-Baker that jumped in after World War II and created a design that quickly filled the needs of most Western air forces, including the RAF (British Royal Air Force).
The U.S. Air Force long insisted on using only American made ejection systems but the U.S. Navy stayed with Martin-Baker because the American ejection seat did not function as well at very low altitudes (where a lot of naval aviators have to eject during carrier operations). Martin-Baker supplies about two-thirds of the ejection seats for Western fighter aircraft including the F-35. The other major supplier of ejection seats was the Soviet Union. Those Soviet era manufacturers continue to produce good ejection seats for Russian aircraft and some foreign customers. China is becoming a major player in this area, usually exporting Chinese made ejection seats in Chinese made aircraft. The Czech Republic and Romania also manufacture lower end ejection seats. Western manufacturers produce about a thousand seats a year, while Russia and China produce less than half as many, almost all of those seats are for locally made aircraft.