The issue is that the cost increase is not proportional.

If you go from an Economy Basic seat to a Economy Plus seat I'll pay 125% more. Meaning a $39 seat now costs $90. Does it cost over 100% more to fly an over six foot person than a five foot eight person? No...

no, there would be different seats for different people, smaller for shorter people, bigger for taller, capacity would stay same and everyone would have same level of comfort

currently i pay same price as my 36cm shorter wife, so she has luxurious leg space and i suffer. it could be easily resolved if her seat would moved closer to seat in front of her and they could seat someone tall behind her. now i would be fine paying more becausei am heavier than she and there is nothing we can do about it but that would be at least rational, pay for each kg same as with luggage, but current system giving short people lot of space while they pay same as me and i suffer is not fair and i am subsidizing them with my leg space, while they get extra inches they don't need because airlines are not flexible