A person's body weight and body type are factors which have an impact on blood alcohol concentration.  In general, the less you weigh the more you will be affected by a given amount of alcohol because the alcohol has less room to spread out, making the concentration higher.  As detailed above, alcohol has an affinity for water, so the more water there is in which to distribute the alcohol, the lower the blood alcohol concentration will be.  Basically a person's blood alcohol concentration is a function of the total amount of alcohol in the person's system divided by total body water.  Therefore, if two individuals with different weights but similar body fat compositions consume the same amount of alcohol, the larger individual will achieve lower alcohol concentrations than the smaller one because there is more room for the alcohol to spread out.

To illustrate this point consider the following: imagine a drop of blue dye is put into both a shot glass and a gallon jug which are both full of water.  The concentration of blue dye per unit of measurement in the smaller container (the shot glass) will be greater (the water will be more blue) than in the larger container (the gallon jug) because there is less space for the dye to spread out.  Larger people are simply larger containers than smaller people, so assuming the same quantity of alcohol is ingested, smaller people will be expected to reach a higher blood alcohol concentration than larger people. 

Similarly, if two people of the same weight consume the same amount of alcohol, a person with a higher percentage of fat will be reach a higher peak blood alcohol concentration than a lean, muscular individual because fatty tissue does not contain very much water and will not absorb very much alcohol, making the concentration in the rest of the body higher for the person with the higher percentage of fat.