title_body: Why do some beers seem to make people feel more full?
upvoted_answer: I think a some of it is psychological - I too remember struggling to drink down a pint of Guinness. But now I can put away plenty!  The darker color and dense head give the perception of a bigger beer.
  
  But in reality most pub Stouts such as Guinness are quite light beers, in terms of their physical density (specific gravity) so it's not that they are physically harder to drink.
  
  Some reasons that Stouts can taste and feel heavier:
  
  
  Stouts tends to be more bitter, so tougher to drink for those not used to it
  Serving on N2 ("Nitro") creates the illusion of a thicker mouthfeel (it's not fully understood at present quite why that is)
  The lower CO2 levels removes a certain "light" quality from the beer. Beers with more CO2 feel lighter, effervescent and easier to drink.
  
  
  Of course, there are some stouts that really are fuller bodied, but I focused on Guinness simply because that's the one most available throughout the world and yet is counter-intuitively lighter than typical pub beers.