When baking cupcakes and muffins, there are two reasons why the batter has to be divided equally among the holes in the muffin pan.
First, you want your cupcakes or muffins to be the same size.
Second, and this is related to the first, because you want your cupcakes or muffins to bake evenly. If some holes have less batter than others, the cupcakes or muffins in those holes with yield smaller cupcakes or muffins and they will be overbaked before the cupcakes or muffins in the holes with more batter are done.
So, you want the baked goodies to be the same size. Which means you have to fill the holes equally. How?
When the batter is thin (pourable), I use a measuring cup. One-fourth or one-third measuring cup for large muffins or cupcakes.
When the batter is thick and cannot be poured, I use an ice cream scoop — the kind you press to release the ice cream or, in this case, the batter (note that ice cream scoops do come in different sizes so you just need to choose which fits the capacity of the holes in your muffin / cupcake pan).
Because you’re actually measuring the batter at the same time as you’re filling the holes, the batter is divided equally (well, more or less) and the baked cupcakes or muffins rise evenly, brown evenly and are of the same size.
And when you serve the cupcakes, everyone gets equal share. In my family, that’s very important. Even the amount of frosting must be equal; otherwise, there are complaints.