Why is mRNA not a macromolecule?
A macromolecule is a large molecule made up of smaller building blocks. There are 4 classes of biological macromolecules: protein, lipids, carbohydrates and nucleic acids. By this definition only, the answer would be nucleic acids.
But RNA is one of two types of nucleic acids, so it is also a macromolecule.
Here’s some quotes to back me up:
Ribonucleic acid, or RNA is one of the three major biological macromolecules that are essential for all known forms of life (along with DNA and proteins). (RNA society)