Interesting one this, there is a known issue around the removal of LOB data in sql server. Dropped LOB data is not removed immediately, there is a ghost process that runs in the background and removes the ghosted records.

Truncating the table as far as i remember will clear the table but i couldn't say for sure when just dropping the column.

Once you drop the column have you tried querying the system catalogs to see what allocated, used and reserved space sql server thinks the table\index object has?