filled to the brim

i went to lunch at chipotle the other day, as you can see by the picture, and when i opened up my burrito i was amazed by the sheer size of the thing.  i thought to myself, "how can any one person possibly eat all of this food in one sitting?" i barely made it through half of it before feeling like i was going to pop at any second.  this led me to start thinking about portion size and how much it has changed over the years and how this must be part of the cause of today's obesity problems. i did some searching and i came across an article on the centers for disease control and prevention website. (http://www.cdc.gov/nccdphp/dnpa/nutrition/pdf/portion_size_research.pdf)  in the article, it states that portion sizes have, in fact, grown over the past two decades.  the article states that, "americans are surrounded by larger portion sizes at relatively low prices, appealing to the consumer's economic sensibilities."  i continued reading the article and one particular part basically jumped off the page at me and left me dumbfounded.  the article states, "there are no empirical studies to show a causal relationship between increased portion sizes and obesity, but there are short-term studies showing that controlling portion sizes helps limit calorie intake, particularly when eating high-calorie foods."  it took me awhile to pick my jaw up off the floor and wipe up the drool, but then i started thinking about how there is absolutely no way that the increasing portion size isn't a direct in obesity.  it just seems like plain common sense that people are going to tend to overeat when they are presented with a portion size that is much greater than the suggested serving size.  i just know that if i had the stomach for it, i would have finished my whole chipotle burrito bowl in one sitting because it tastes that good.  i guess it just baffles my mind that to this day, there is only short-term evidence, as stated in the article, that shows that people eat more when they are confronted with larger portion sizes.  i understand that other factors contribute to obesity, but i do not understand how a direct causation between the two has not been found.  i can't be the only one that thinks this is common sense and pretty much on the level of a high school biology paper, can i?