Obviously a heavier person requires a greater dose of anything to have an equal effect. But it is not clear that a bigger person will have more cells necessarily, if the only difference is fat cells. This is because fast cells get bigger when you out on weight, you don't necessarily make more fat cells. This is probably one of the differences between children, who rarely become obese, and adults, who do so easily with a bad diet.
My general take is the bigger you are the more dose you need.
I also think that all things being equal, larger people (not just with fat but height and muscle as well) will have shorter telomeres. This is obscured in the cross sectional studies by different inherited telomere lengths, lifestyle, etc., but it is most likely true - when is the last time you saw a super centenarian who wasn't small?