Or consumption in our case here.
Reading over on Pete Brown's blog that a study has found that people are overestimating the amount of calories in a pint of beer and beer actually has far lower calories. Brings me to my usual point that beer or food for that fact doesn't make folks fat, but not actually knowing what both the calories are and the correct portion size for that amount of calories.
Packets of food now have little calories and percentage of intake guides on them but how closely do folks read them? That packet may actually say (in small wording) that a third of a packet gives you X calories (in larger wording). So if you don't read it properly instead of glancing it is not unlikely that you will assume the whole packet is X calories.
Many work out calories for alcohol based on 100ml but don't realise what a pint is in ml, I have heard people say in my earshot that a pint is about "800ml"..... oh really......?
The main reason though for beer or any alcohol causing weight gain though is the fact it makes you hungry, those pub sized packets of crisps are more tempting or that sandwich at home is calling out to be two.