I’ve wondered this for years. I stand 6’4" and wiegh 230#. Let’s say you compare me to a woman standing 5’6" and 115# How the hell can we both be healthy on the same amount of vitamin-x? Are my cells bigger and therefore have the same number? Do I have more cells and therefore need an extra shot to adeqately cover the needs of each cell? I know the RDA is still under debate, but how can the same pill cover the needs of such differing body types?
I think an exact calculation of how much of a particular vitamin or mineral a person needs would require knowing their exact weight, as well as gender, age, and activity level (an active 70lb 10 year old, for instance, may require more of some nutrients than a 200lb 40 year old couch potato)
The RDA’s put out by the government are based upon a mythical creature, the “average human being”, plus a slight fudge factor to allow for various factors like bigger than average people, slight degradation of vitamins over time, and so forth. Some of the smaller folks are likely getting an excess, but not enough to cause problems (much of the excess will simply be excreted)
Some items can cause an overdose - adult sized iron pills, for instance, can be extremely hazardous to toddlers, and excessive amounts of vitamin A are toxic to anyone, even fatal if the dose is high enough - which is one reason why one should not take mega-doses.