Fantastic topic, I have been wondering this myself.
When you think about it, it makes sense. A larger person has more bodily fluids and tissue, and when the drug is dissolved and absorbed into the bloodstream, it's dispersed throughout the body. So a person with more blood, and more body mass, should theoretically (at least this makes sense to me) have a different ratio of the drug to a given amount of blood, meaning higher serum levels of the drug. It's the same reason why a 105 pound woman gets drunk off of a beer or two when a 250 pound man might not even catch a buzz.
Say you dissolve 1mg of Finesteride in 5 pints of water, and another 1mg into 8 pints of water. Take an 8 ounce glass from each, and you've got higher levels of Finesteride in the glass taken from the 5 pint sample. I'm pretty sure that this is why most over the counter medications give you a range of dosages, they'll tell you to take one, and then if it doesn't have much of an effect, take another one, up to a maxumum dose per day. Small doses for children are given based on their smaller bodyweight (and possibly higher sensitivity to drugs in general, but I'm not sure about that). Even for adults, it's usually recommended to take the smallest effective dose.
I'm only 5'9" and and 150 lbs. I get terrible sides from propecia 1mg, and I still get them, albeit not as bad, when I break them in half. I also experience this with other drugs as well, I usually only have to take the minimum dosage to get good results.
I stopped taking propecia about 9 months ago after my inability to deal with the sides, but my hair is thinning like crazy and I'm considering trying it again at .25 mg, by quartering up the 1mg pills. If it's true about the .2mg dose being almost as effective as the 1mg in most men, then I suppose it's worth a shot.
I'm not a doctor or a biochemist, but I'm pretty sure it's the concentration of drug in a given amount of blood, and not the total amount of drug in the body, that matters.