Re: It is not portion size which matters
You're on the right track and bread is a key factor. You can go further and say carbohydrates in general contribute to obesity, diabetes, and a host of other conditions known collectively as metabolic syndrome. This has been shown in numerous high quality studies and meta-analyses. The biochemistry of this is very simple. The body considers carbs as a rarity because for most of the past million years we haven't had supermarkets with aisles full of cereals, crisps, sugary drinks, bread, baked goods, biscuits, jam, sweets, etc. So when you consume carbs the mechanism that makes you feel full is turned off so that you can carb-load and build up fat reserves quickly. That's fine if it's for a short season before winter, but not every day all year round. Eating food high in natural fats will make you feel full properly and it's difficult to over-consume.
Smaller portion sizes will make no difference now that people are accustomed to snacking (and adverts tell them it's good and healthy so to do). They'll still have the mid-morning and mid-afternoon crash in blood-glucose levels and feel 'hungry' so reach for more sugar. Expect sales of snacks and sugary drinks to increase if this is implemented.
Low carb, high fat is what we always used to eat.