There are 100 of species of birds which live in different habitat and they are of different size. Now the common thing in all these birds is, they all lay eggs. Like they migrate or go from one place to another but there eggs cant do the same. So basically before laying eggs these birds make sure that they have a comfy and cosy palace to lay there eggs. They also make sure that the place should be matching there required habitat and its safe. In that perfect place for there eggs they make their nest where they can lay there eggs. After they lay there eggs in that nest they protect those eggs. Till the eggs are not broke and a baby bird came out of it they give their body warmth to the eggs. Once the baby birds comes out of the eggs, they still live in that same nest for few months, so that the mother can teach her children how to fly, eat, catch food, etc skills for living. Once they are big enough to live on their own they all fly away.
It takes time to build a nest. As birds needs to pick many stings, feather and other comfy stings for them to weave those things together and make a comfy nest of them.
Now as we already know birds are of different sizes, so of course the size of there eggs will depend on the size of their parents.
Example- An ostrich egg weight around 1600 – 2500 gram. They can lay 60 eggs at a time. So just imagine the size of the nest of an ostrich, its really big.
Where a humming birds eggs weight is around 2.5 gram. So they only need a small nest.