Eating smaller portions helps a lot to lose weight. We are eating 30% more than we need, each meal. But, I can tell you that it is not easy to resist the temptation of another bite.
One tip is to eat slower than you usually eat. This action will help you to feel satisfied and stop eating.
Therefore, portion control is important when you're trying to lose weight and keep it off.
These websites have more content about this topic.
Slide show: Portion control for weight loss
Portion Size and Weight Loss
How to Avoid Portion Size Pitfalls to Help Manage Your Weight