Database disk overhead

I have telegraf recording stats on influxdb (1.8…0-1). When I view these in a Grafana graph, they show my total database sizes to be about 600MB. However, the data files are taking up 38GB,

The database sizes as reported by telegraf stay consistent, but the disk usage just keeps going up. I do have retention policies set on all my databases.

Is there some problem in my setup, or is this expected behavior?