I am not a doctor, and this is not medical advice.
A proper, correct dose (total amount) of drug is determined by the dosage (amount per body weight) and the weight of the individual. It also should account for individual variations in sensitivity.
Mostly, to save time, doses are determined by multiplying the dosage by an average body weight. For most drugs, which have a very broad dose / response curve, and thus a wide range of therapeutically useful values, that's fine. For some drugs, you have to be more careful.
My wife (yes, this geek got lucky) is quite small, and thus nominally takes about 1/2 of prescribed doses (to be entirely accurate, she should be taking about 2/3, but most drugs have a broad dose / response curve, and 1/2 a pill is easier to create than 2/3 of a pill). I'm about average weight, and so the nominal doses typically work fine for me. Over-the-counter drugs have doses (again, total amount) based on a 170 lb body weight; it's pretty easy to normalize by 170 to get the dosage (again, amount per body weight) and multiply by your weight to get a more accurate dose. If you weigh anything close to 170 lbs, it isn't worth the effort.
This is not medical advice, I am not a doctor, and you should take the preceeding only as interesting information. What you do with it, and any consequences, are entirely your responsibility.