First check which tables can account for the unexpected growth and then try to corrrelate this to available functionality that could influence this.
Also check if the growth is still present after restoring the database locally. If the size has reduced significantly you might be dealing with bloating and need to identify the code that is responsible for creating/updating/deleting data excessively.
Tip: Try google for finding SQL statements for table sizes and bloating, they are readily available.
Many thanks everyone
To see an actual reduction in the database after deleting records will need to recover the physical disk space.
Reducing Database Size | Mendix Documentation