SQL Server – Table Size Increase After Column Data Type Change

When you delete from heaps, allocated space may not be unallocated unless you use a table lock. See "Deleting Rows from a Heap" from DELETE on MSDN

This is a separate problem to fragmentation (which happens of course)

Just in case, have you tried this to update usage info?

EXEC sp_spaceused 'MySchema.MyTable', 'true'

With no indexes or keys, you can't defragment it normally.
You can add/drop a clustered index, or do something like this

SELECT * INTO [MySchema].[MyTableNew] 
FROM [MySchema].[MyTable]
-- ORDER BY something

-- if you like
EXEC sp_spaceused 'MySchema.MyTableNew', 'true'

DROP TABLE [MySchema].[MyTable];

EXEC sp_rename 'MySchema.MyTableNew', 'MyTable';

oops: just seen updates, my Internets has been broken on and off...

When using the DBCC SHRINKDATABASE statement, you cannot shrink a whole database to be smaller than its original size. Therefore, if a database was created with a size of 10 MB and grew to 100 MB, the smallest the database could be reduced to is 10 MB, even if all the data in the database has been deleted.

However, you can shrink the individual database files to a smaller size than their initial size by using the DBCC SHRINKFILE statement. You must shrink each file individually, instead of trying to shrink the whole database.