I've Moved the clustered index on large tables to new filegroups to defrag, compress, and to recover space more easily from the original "primary" filegroup.
A 315GB compressed Clustered Index (page compression) resulted in a filegroup that expanded to over 650GB. (Uncompressed, this table is north of 2TB.) The free space is not limited to the end of the file, so it cannot be truncated - to shrink this file will be slow and likely result in increased fragmentation.
Let me know if you know how I can create this index without the excessive growth.