If you think much about the different platforms, it makes perfect sense that there are specific and varied requirements. Presumably they're pretty flexible about who flies a C-5, considering it's big enough to carry Chinooks or M-1 tanks [0]. OTOH, ejecting out of a fighter jet probably doesn't go very well if your knees are smashed up against the dashboard.
Sitting height is just as important for safe ejections as leg length. My dad was 5'10" but with a tall sitting height and he was just barely under the safety line for a seat in an S-3.
Having short limbs and a long body can indicate the presence of a medical condition known as "Hypochrondroplasia".
> Hypochrondroplasia is a genetic disorder characterized by small stature and disproportionately short arms, legs, hands, and feet (short-limbed dwarfism). Short stature often is not recognized until early to mid childhood or, in some cases, as late as adulthood.
[0] https://en.m.wikipedia.org/wiki/Lockheed_C-5_Galaxy