I'm having the same problem on a table with a varbinary(max) column. I have 21 rows at 2 MB in the varbinary(max) column totaling about 44 MB for the table but sp_spaceused shows 310 MB used. Even update statistics or reindexing doesn't change that number. If I copy the data to a new table, it will show 44 MB for the new table. In other words, I don't think reindexing is moving the off page data nor calculating its true size.