Why might a table's data space take up 4x the size of the raw data?
After discussions in the comments on the original question, it appears in this case the lost space is caused by the choice of clustered key, which has led to massive fragmentation.

Always worth checking the state of fragmentation via sys.dm_db_index_physical_stats in these situations.

Edit: Following update in comments

The average page density (prior to rebuild of the clustered index) was 24%, which fits perfectly with the original question. The pages were only 1/4 full, so the total size was 4x the raw data size.