Deleting data will cause the backup size to decrease but the file sizes of the .mdf files will not. However, you will fragment the data within the tables you delete from so rebuilding the indexes will make the total used within the data file get smaller as the clustering index will clean up all of the holes you created by deleting data. HOwever, the file size of the .mdf won't get smaller. Like others said you will have to use the SHRINK command to do this. However, do NOT run this on a heavily used db, run it during a quiet time as it could potentially lock people out. Also keep in mind if you shrink the .mdf you are now fragmenting the data within the database so you should probably rebuild the indexes afterwards.

A lot of folks here would say do NOT shrink your files because you cause more fragmentation within the file and it will grow again as you add more data so you aren't gaining anything. About the only reason to shrink a file, some would say, is if your drive is dangerously about to run out of space.