I'm going to assume you're talking about the Roman military organisation and not something else.

The Romans in fact never really had centuries consisting of 100 men. Yes, the 'cent' in century means "hundred", but this was simply a name which had stuck from earlier times.

During the late monarchy (6th century B.C.), a full military levy of hoplites was 6000 men, consisting of 60 centuries of 100 men each. After the end of the monarchy and the establishment of the Roman Republic, there were two consuls leading the army instead of one king. The levy of men was divided equally among those two consuls. So the legions halved in size, but the number of centuries was kept at 60, which meant that the centuries also halved in size.

Over time the Romans came to a system where a legion was divided into 30 maniples, each maniple consisting of 2 centuries of 60 men (first two lines) or 30 men (third line). This system was used from about the 4th century to end of the 2nd century B.C.

In the last decade of the 2nd century, the organisation of the legion was changed. Instead of 30 maniples in a legion, there would be 10 cohorts of 3 maniples. The cohort would be the new main tactical formation. Each maniple still consisted of 2 centuries, but now a century consisted of 80 men (with no difference between lines).

Do you mean in a Roman legion's century?