Many possibilities. Many factors contribute to the effect of any drug including alcohol. To start with the purely physico-chemical: while being obese somewhat dilutes a drug, ones that are very much associated with water and not oily tend to be distributed in the blood and not so much the entire body. For this reason, size (height) is a bit of a better predictor of blood level than total body size. The rate of alcohol absorption has a huge effect and taking alocho on an empty stomach or with grapefruit juice increases how much alcohol is absorbed along with increasing the speed of absorption. Then, several drugs and conditions lower the liver metabolism of alcohol and drinks build up more. This would not be an effect seen so much with one drink that is low in alcohol. Then there are the drug and other dynamic interactions. Other things taht make one tipsy iwll add up with alchol to make one more tipsy faster. Other drugs such as muscle relaxants and any that work on the psyche are likely to add into the effects of the alcohol. Then, there is the purely psychological. One can be conditioned to expect a drug effect and then one will have that effect even if the drug is not actually given. This is mostly unconscious and automatic.