Is portion size to blame for obesity?

I've often seen presentations where the authors show the size of an average hamburger or soda in the 1960s, 70s, 80s, and 90s as a way of making the point that portion sizes have increased, and thus contributes to the rise in obesity.  Changes in portion size probably have played some role, but at least according to this experiment published in the journal Obesity, it may play less of a role than first meets the eye. The researchers recruited over 233 people working in a large medical complex and randomly assigned them to treatments that differed according to the size of the free lunch they were given (one control group was given no free lunch at all).  Here's what they found:

Adults (n = 233) were randomly assigned to one of three lunch size groups (400 kcal, 800 kcal, and 1,600 kcal) or to a no-free lunch control group for 6 months. . . .

Body weight change at 6 months did not significantly differ at the 5% level by experimental group (1,600 kcal group: +1.1 kg (SD = 0.44); 800 kcal group: −0.1 kg (SD = 0.42); 400 kcal group: −0.1 kg (SD = 0.43); control group: 1.1 (SD = 0.42); P = 0.07). Weight gain over time was significant in the 1,600 kcal box lunch group (P < 0.05).

A remarkable increase in portion size from 400 kcal to 1600 kcal for lunch over a 6 month time period resulted in no statistically significant differences in weight across groups at the end of the period.  If you compare the pre- and post-weight of the people in the 1600 kcal group, there was a slight increase (0.19kcal/month) in weight for people in that group but not for people in the 400 kcal group or the 800 kcal group.  Curiously, those in the control treatment, which included people who were not given a free lunch, gained a statistically significant 0.24 kcal/month - more than those in the 1600kcal free lunch group! 

The trouble with many "interventions" such as this (similar to those that happen at school lunches) is that people can substitute across time.  If I eat a big lunch, I'm likely to eat less for dinner. And vice versa.  If I eat a 400 kcal lunch, I'm more likely to grab a snack in the afternoon than if I eat a 1,600 kcal lunch.  

Apply this thinking to related policy ideas.  Ban sodas or transfats. What will people drink and eat instead?  Tax restaurants.  What will people eat at home?  Add more veggies to the plate at school.  What will happen to veggie consumption at home?  I'm not saying that such policies might not have some effect on weight, only that because of substitutes and compensating behavior, they will often have less effect than is expected.