Why do heavier people have a lower blood and alcohol level?

In general, the less you weigh the more you will be affected by a given amount of alcohol. Alcohol has a high affinity for water. Basically, one's blood alcohol concentration is a function of the total amount of alcohol in one's system divided by total body water. So for two individuals with similar body compositions and different weights, the larger individual will achieve lower alcohol concentrations than the smaller one if ingesting the same amount of alcohol.

However, for people of the same weight, a well-muscled individual will be less affected than someone with a higher percentage of fat since fatty tissue does not contain very much water and will not absorb very much alcohol.