DROP COLUMN doesn't physically remove columns

DROP COLUMN only makes the column invisible to SQL operations by setting attisdropped=True in the system catalog table pg_attribute. Existing rows keep the original value, while new rows will store NULL on that column.

Thus, dropping a column is quick, but it doesn't reduce the disk size of your table. That space will be reclaimed over time as existing rows are updated. However, if you want to reclaim disk space immediatelly, you can execute VACUUM FULL to force a rewrite of the table.

See more info on StackOverflow.