Bonglebear,
It's those damn accountants every time you see. Pilots at the start of their career are cheaper because, as you have pointed out, the pilot shortage makes them smaller, fitter and thinner. Thus cost less fuel to cart around and monitor the auto pilot.
As the pilot gains more experience, eats more crew food, drinks considerably more coffee (unless you fly for easy!) then you become more expensive to the company payroll as you get fatter, thus heavier, thus burning more fuel.
It's all in the demographics.