I understand why this is being downvoted, and agree that it's not a nice point to try and argue - however it is somewhat irksome when I (as a recovering anorexic with a very small physical footprint) is penalised for a baggage weight allowance and yet there's no difference in seat cost between myself at 51kg and someone at twice that.

Completely appreciate there's no fair way to police it (other than that, I think Samoan airline, who charged based on the person/bag weight combination), but I think it's indicative of it all being a bit of a sham. Of course weight increases costs of flying, but is an extra 1kg in a checked bag really worse than carrying 20/30kg more of fat or muscle?