Shalu Ganotra Chadha, et al, explain how to keep your Delta Lake tidy:
The useful features of Delta Lake come at the cost of requiring regular maintenance. Delta Lake requires periodic cleanup as it accumulates files over time with each upsert and retains previous snapshots of the data. They can quickly convert a small dataset (in MBs) to several GBs of storage. This is because deleted data is not really removed but retained as an older snapshot of the Delta Lake dataset.
Click through for two operations you can perform on a Delta Lake, as well as some recommendations on when to do what via the Genie Delta Lake Auto Maintenance scripts they provide.