Here's why:

* CO2 (Carbon Dioxide): Consists of one carbon atom and two oxygen atoms. It's a relatively simple and small molecule.

* Amino Acid: A much more complex molecule with a central carbon atom bonded to a hydrogen atom, an amino group (NH2), a carboxyl group (COOH), and a variable side chain (R-group). The R-group can vary in size and complexity, contributing to the overall size of the amino acid.

Therefore, the smaller size of the CO2 molecule compared to the amino acid molecule is due to its simpler structure and smaller number of atoms.