I really wonder why breakfast is the way it is, because since it's the first meal of the day, you'd think a big breakfast (e.g. dinner) would make sense as you need the calories during the (work)day. But maybe breakfast is relatively light because a lot of people have some digestive issues or sensitive stomachs in the mornings.
Circadian rhythm. In many people their liver releases a bunch of glucose right before waking, something diabetics term the "dawn effect" and need to control for. This helps get the metabolism going first thing in the morning (at least, in non-diabetics) and delays the need for caloric intake until later in the day. This effect is easily witnessed if you check your blood glucose before bed and when you rise in the morning: for example earlier in the week mine was around 5.4 mmol/L (normal) at bed time and 8.6 mmol/L (high for fasting blood glucose) before breakfast the next day.
A normal person will need caloric replenishment during the day, especially later in the day before (typically) an 8 to 10 hour fast during the night.
So, small proteine-heavy breakfast, large balanced dinner in the evening with a smaller balanced mid-day meal in the middle.
It depends on whether you're making the breakfast, or someone else is making your breakfast.
Light breakfasts are popular if you have to make it yourself
Eating a lot of food is tiring and you have plenty of energy stored from the previous day(s) food.