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.

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.