I know several people who refuse to drink stouts, or will only have 2 as opposed to 4 if we were drinking something else (generally a bock or lighter beer). I feel like I drank less of them too, and it is always to the same end; feeling too full to drink another.
I remember looking it up at some point and the Calories & ABV were negligibly different.
There must be a reason, I know several people who feel the same way. Since then I've just assumed that it is psychosomatic and that the fuller taste makes you feel more full.