One of the biggest problems in Western society are the MASSIVE portions of food we assign ourselves. You may have seen diagrams such as the one below showing our steady increase in serving size over the years.

Though your first reaction may be “pfft look at their tiny meals! What a rip off!” you may change your tune when you read the fine print. The fact is we in the first world live with such excess that we have lavish amounts of everything and the commercial world has capitalised on that by increasing the average portion size. We eat more, we buy more.
Weight-loss and even maintaining weight is, at it’s core, the difference between the calories you put into your body and the calories you expend in the space of a day. It’s why exercise works - burns more calories - and it’s why, if you’re eating these massive portions you can gain weight even with healthy food. Your plate should never be full (except on special occasions…).