few kafka-logs directories are utilizing lot of space

In our ETD environment, 2 of the kafka-logs directories are utilizing lot of space. The retention period of kafka-logs is set to 12 hours and we cannot find any log file older than 12 hours.

Details of filesystem occupying more space:

/kafka/kafka-logs/SID_SID-hana-NormalizedDataOut-X

/kafka/kafka-logs/RTLogEventIn-X

Please help with below queries:

1. What factors affect the size of these 2 directories?

2. What's the impact of reducing the retention hours to a lower value like 8?