Seat belts aren’t a standard length, and standard maintenance practice for fixing them is simply to shorten them beyond wherever the damage occurred. On some aircraft, I’ve seen a single seat’s seatbelt be dramatically shorter than the norm.

Even with a BMI of 26-27 - mildly overweight, but not at all obese - I once ran into an errant first class seat with like a 6” seatbelt length and needed to request an extender.

It very very rarely happens, but I’d be furious if I got kicked out of an exit row despite being well within the normal size range because maintenance took the lazy way out. Since lengths aren’t standardized, and the extender doesn’t change the difficulty of exiting the seat quickly, I imagine it’s not really a useful metric as a safety rule for exit row qualification.