compressing only to find less space than before!

I have compressed a .hdd files only to see the hdd space in my macbook dropped. How come and how to resolve this?

I also encounter a number of error messages when I try to compress files in my 3.0 backup created by 'backup and convert' during the upgrade to 4.0. The error messages are to the effect that the files to be compressed are in use or there is an error trying to compress. Again how come and how to resolve them?

Thanks.

 

open Console application and search system log for disk* entries,
Can you see anything related to disk*
not * means your particular configuration number and slice number