Why do low-molecular-weight amines often have fishy or putrid odours?

Lower-molecular-weight amines are characterized by distinctive fishy to putrid odours because they are highly volatile compounds that exist as gases or easily vaporized liquids at room temperature.1

These odours are a defining physical property of smaller amines. As the molecular size of an amine increases, its volatility decreases, causing the intensity of the odour to diminish until it eventually becomes unnoticeable. Notably, specific diamines such as putrescine and cadaverine are well-known for their foul, putrid smells, as they are produced during the process of decaying flesh.1