“Lightweight pilots” may be a euphemism for “female pilots”.
Yeah it totally is. My older sister use to fly Harriers with the USMC. While she was doing that a smallish male pilot died during an ejection and they did an updated study on its seat. They discovered it was going to kill her if she had to eject at low speed. The seats have different thrust modes and when you are low and slow they kick harder to get you higher... and people who are smaller get more acceleration (F=MA, F is constant and M goes down) also smaller people (I.E. women) have less bone density. Some newer seats have different engines and more intelligent thrust modes. Had my sister been flying F-18s she would have been ok. Instead she stopped flying Harriers and switched to tankers.
Why not add some ballast to the underweight pilot to reduce the ejection speed? Lead britches?