One of my databases is 25GB, the compressed backup is 2,9GB. Another one, database 105GB, compressed backup is aprox. 20GB.
And still on truncate, an interesting view you can read about here:
And I find this really interesting, didn't know about it myself until now :w00t:
When tables require that all records be deleted and TRUNCATE TABLE cannot be used, the following statements can be used to delete the data and reset the identity value:
DELETE from "table_name"
DBCC CHECKIDENT("table_name", RESEED, "reseed_value")