The human body can only digest a certain amount of food at any one sitting.

For example protein is important for someone trying to put on more muscle mass for body building but there is no point filling a whole plate up with meat or eating two cans of tuna at the same time as most of it will go to waste.

Excess energy and calories that are not burnt off during the day will get stored as fat overnight which slows your metabolism down.
Breakfast is the most important meal of the day as it ‘breaks the fast’ that your body goes through when you are asleep. From your evening meal to the following lunchtime could be anything up to 15 hours without food. If lunch is your first meal of the day your metabolism will be so slow that your body will only digest some of it and you will be storing up some unspent calories already.
Breakfast kick starts your metabolism and with healthy small meals and snacks throughout the day (making sure you don’t over eat) your body will be constantly trying to digest your food keeping your metabolism high and burning calories.
One way to look at portion sizes is to use your hand. One serving of protein should be about the size of the palm of your hand. One serving of green vegetables and salad greens could be the size of two fists, while starchy items like potatoes or pasta should be served in a portion about the size of one tightly clenched fist.
These healthy portions may seem small at first, but realise that your stomach is only about the size of your two loosely fisted hands held together. Visit my above post for tips on controlling portion sizes.